Remove the legacy AST codegen — IR path is canonical now

The `--use-ast` path through `src/codegen/mod.rs` was a strictly
inferior subset of the IR codegen. Building every example with
`--use-ast` through the jsnes harness:

- `arrays_and_functions` — fully black (array init + function
  return values + OAM-in-loop all broken)
- `structs_enums_for` — fully black (struct literal is a no-op,
  all fields stay at 0)
- `inline_asm_demo` — fully black
- `bitwise_ops`, `loop_break_continue` — below sprite floors
  (static `next_oam_slot` bug B)
- `match_demo` — panics at compile time with
  `branch offset 153 out of range` (AST's if/else-chain
  desugaring of `match` emits short branches that can't reach
  the far arms in a multi-arm match)

Six of fourteen examples are non-functional under `--use-ast`.
The other eight happen to fall inside the subset AST handles
(no arrays, no structs, no function return values, no
multi-sprite loops, no long match chains).

`docs/future-work.md` already listed "Once working, delete the
AST-based codegen entirely" as the intended direction. It's
working, so this commit does the deletion.

What's removed:
- The `CodeGen` struct, its impl block, and every helper in
  `src/codegen/mod.rs` (the AST codegen body) — ~1150 lines.
  The file is now a module header that re-exports `IrCodeGen`.
- `src/codegen/tests.rs` — 15 AST-specific instruction-pattern
  tests. Every feature they covered has an equivalent test in
  `src/codegen/ir_codegen.rs::{tests,more_tests}` already.
- The `--use-ast` CLI flag and its branch in `src/main.rs`.
- `compile_with_ir_codegen` in `tests/integration_test.rs` —
  `compile()` now does what it did, so they merged. All 40
  integration tests go through the IR path.
- Outdated sections in `docs/future-work.md` that described the
  IR codegen as "not yet implemented" and listed AST codegen
  gaps as priority work.

What's kept:
- `src/codegen/ir_codegen.rs` — the real codegen.
- `src/codegen/peephole.rs` — post-codegen cleanup pass, now
  run unconditionally from `main.rs`.

Test plan:
- `cargo test --release` — 313 unit + 37 integration tests pass
  (was 328 + 37; the 15 dropped are the deleted AST-specific
  tests).
- `cargo fmt --check` clean.
- `cargo clippy --release --all-targets -- -D warnings` clean.
- `node tests/emulator/run_examples.mjs` — 14/14 ROMs render
  above their per-example nonBlack floors.
- The one tightening: `sprite_resolution_uses_tile_index` was
  asserting on the old static-slot encoding
  (`A9 01 8D 01 02`). Updated to the cursor-based form
  (`A9 01 99 01 02`, i.e. STA AbsoluteY).

Net diff: 1581 deletions, 62 insertions.

https://claude.ai/code/session_014Z5y3Q9krLcAxYpZQJhZ5V

docs/future-work.md

@@ -7,64 +7,9 @@ in the NEScript compiler. Items are organized by priority and area.
 
 ## 1. IR-Based Code Generation
 
-**Status**: The IR pipeline (lowering + optimization) runs during compilation but
-its output is discarded. Code generation still works from the AST directly
-(`src/codegen/mod.rs`). This means IR-level optimizations have no effect on the
-final ROM.
-
-**What exists**:
-- `src/ir/mod.rs`: Complete IR type definitions (`IrProgram`, `IrFunction`,
-  `IrBasicBlock`, `IrOp`, `IrTerminator`)
-- `src/ir/lowering.rs`: AST → IR translation for all statement and expression types
-- `src/optimizer/mod.rs`: Constant folding, dead code elimination, strength
-  reduction, ZP promotion analysis, function inlining — all operating on IR
-
-**What's needed**:
-- A new `src/codegen/ir_codegen.rs` that walks `IrProgram` and emits 6502
-  `Instruction` sequences from `IrOp`/`IrTerminator` instead of from AST nodes
-- Register allocation strategy for IR temps → A/X/Y/zero-page spill slots
-- Replace the `CodeGen::generate(&program)` call in `main.rs` with
-  `ir_codegen::generate(&ir_program, &analysis)`
-- Once working, delete the AST-based codegen entirely
-
-**IR lowering issues to fix first** (found during code review):
-- `ButtonRead` emits `ReadInput` with no destination temp, then uses uninitialized
-  temp in the `And` mask operation (`src/ir/lowering.rs:534`). The fix: `ReadInput`
-  should store the input byte into a temp, or the lowering should emit
-  `LoadVar(t, input_var_id)` after `ReadInput`.
-- Logical AND/OR use raw `VarId(self.next_var_id)` for temp storage without
-  registering it (`src/ir/lowering.rs:603,637`). Should use `IrTemp` instead.
-- Break/continue create unreachable blocks that contain subsequent dead statements
-  (`src/ir/lowering.rs:259`). Should either skip lowering after a terminator, or
-  the dead code elimination pass should handle it.
-
-**Impact**: Enables all optimizer passes to actually affect output quality.
-Currently the optimizer is validated by tests but its results are thrown away.
-
----
-
-## 2. Codegen Gaps (AST-Based)
-
-These features are parsed and analyzed but produce no 6502 output:
-
-| Feature | Location | Status |
-|---------|----------|--------|
-| Function calls | `codegen:148` | `Statement::Call` is a no-op |
-| Return values | `codegen:148` | `Statement::Return` is a no-op |
-| State transitions | `codegen:148` | `Statement::Transition` is a no-op |
-| Array indexing | `codegen:199-200` | `LValue::ArrayIndex` assignment is a no-op |
-| Array expressions | `codegen:417` | `Expr::ArrayIndex`, `Expr::ArrayLiteral` are no-ops |
-| Function call expressions | `codegen:417` | `Expr::Call` returns nothing |
-| Scroll | `codegen:151-152` | `Statement::Scroll` is a no-op |
-| Load background | `codegen:154-155` | `Statement::LoadBackground` is a no-op |
-| Set palette | `codegen:154-155` | `Statement::SetPalette` is a no-op |
-| Multiply/divide/modulo | `codegen:450-452` | `BinOp::Mul/Div/Mod` only emit left operand |
-| Dynamic shifts | `ir/lowering:575` | Shift amount is hardcoded to 1 |
-
-**Priority fixes for a working multi-state game**:
-1. **Function calls**: JSR to function label, pass args via zero-page, return via A
-2. **State transitions**: Write state ID to a zero-page variable, jump to dispatcher
-3. **Array indexing**: Use X register for index, LDA absolute,X for loads
+**Status**: Complete. The AST → IR lowering, optimizer, and
+`src/codegen/ir_codegen.rs` all work end-to-end; the legacy AST
+codegen has been removed. See "Recently completed" below.
 
 ---
 
@@ -373,8 +318,10 @@ These items were documented as future work but have since been implemented:
 - **IR-based codegen** — `src/codegen/ir_codegen.rs` walks `IrProgram` and
   emits 6502 for every IR op: load/store, arithmetic, comparisons, arrays,
   calls, draws, input (P1 and P2), scroll, debug.log/assert, state
-  dispatch, multi-OAM slot allocation, transitions + on_enter handlers.
-  Now the default; `--use-ast` falls back to the legacy AST-based codegen.
+  dispatch, runtime OAM cursor for looped draws, transitions + on_enter
+  handlers. It's the only codegen — the legacy AST-based path and the
+  `--use-ast` flag were removed once the IR pipeline was proven correct
+  by the jsnes emulator smoke test.
 - **IR lowering bug fixes** — `ReadInput` now has a destination temp,
   `ButtonRead` uses the proper input temp, logical AND/OR use a new
   `emit_move` helper instead of the buggy raw VarId temp storage
@@ -490,23 +437,18 @@ These items were documented as future work but have since been implemented:
 
 For someone picking up this codebase, the recommended order of work:
 
-1. **Delete AST codegen** — IR codegen is now the default and beats
-   the AST codegen in 5/7 examples. Once confidence is high, remove
-   `--use-ast` and `src/codegen/mod.rs`'s AST-specific code.
-2. **Struct literals** — `pos = Vec2 { x: 100, y: 50 }` as an
-   expression. Currently fields must be assigned individually.
-3. **u16 / array / nested struct fields** — current structs only
+1. **u16 / array / nested struct fields** — current structs only
    allow u8/i8/bool fields. The layout machinery is ready for more
    types but the codegen only handles 1-byte loads/stores.
-4. **Audio driver** — the `play`/`start_music`/`stop_music`
+2. **Audio driver** — the `play`/`start_music`/`stop_music`
    statements parse but don't generate any code. A minimal NSF-style
    driver running in NMI would unblock game projects.
-5. **Multi-scanline on_scanline reload** — the current codegen
+3. **Multi-scanline on_scanline reload** — the current codegen
    supports one scanline per state but not a chain of handlers at
    different scanlines in the same frame.
-6. **Register allocator** — proper A/X/Y allocation to replace
+4. **Register allocator** — proper A/X/Y allocation to replace
    zero-page spills used by the current IR codegen. Partially
    mitigated by peephole passes but still wasteful in some cases.
-7. **Match statement** — `match x { 0 => ... , 1 => ... }` would be
+5. **Match statement** — `match x { 0 => ... , 1 => ... }` would be
    useful for state dispatch and enum-driven logic.
 8. **Text / HUD layer** — font sheet + layout system for scores.

docs/language-guide.md

@@ -945,7 +945,6 @@ nescript build game.ne --dump-ir
 | `--dump-ir`     | Dump the lowered IR program (after optimization) to stdout     |
 | `--memory-map`  | Dump a memory map of variable allocations to stdout            |
 | `--call-graph`  | Dump a call graph (which handler/function calls which) to stdout |
-| `--use-ast`     | Use the legacy AST-based codegen (default is the IR codegen)   |
 
 ### Check