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- Outdated sections in `docs/future-work.md` that described the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/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