cleanup: fix silent miscompiles and delete dead code exposed by code review

Two correctness bugs were silently producing wrong ROMs:

  - `x << n` / `x >> n` always shifted by 1, regardless of `n`, because
    the IR lowering for `BinOp::ShiftLeft`/`ShiftRight` hardcoded the
    count. Now eval_const the RHS into a compile-time count; fall back
    to a new `IrOp::ShiftLeftVar` / `ShiftRightVar` (runtime loop) when
    the amount isn't constant. Strength reduction folds the variable
    form back to a fixed count once the optimizer knows the value.

  - `x / n` / `x % n` always returned 0, because the lowering emitted
    `LoadImm(t, 0)` for `BinOp::Div`/`Mod` with a comment saying the
    runtime call was "TODO for now". Added real `IrOp::Div` and
    `IrOp::Mod`, wired them through use-counting and DCE, gave codegen
    `__divide`-based implementations, and taught strength reduction to
    rewrite power-of-two divisors into shifts and modulo-by-2ⁿ into
    AND masks. Constant folding now handles `Mul`/`Div`/`Mod`/shifts
    too, which were previously left for the codegen to emit inefficient
    software calls.

Dead code removed (no backward-compat shims kept):

  - `src/debug/` entirely. `DebugSymbols`, `SourceMap`, and the
    Mesen/.sym emitters had no callers outside their own tests;
    `main.rs` never wrote a symbol file. Documented the intent in
    `docs/future-work.md` so it comes back intentionally if needed.
  - `ErrorCode::E0202` (invalid cast) and `E0403` (unreachable state):
    defined, formatted, and marked `#[allow(dead_code)]` but never
    emitted. W0104 now carries the unreachable-state semantics too.
  - `Level::Info`: never constructed.
  - `load_background` / `set_palette` statements and their
    `BackgroundDecl` / `PaletteDecl` parser support: parsed and
    silently dropped by IR lowering (`// TODO: implement in asset
    pipeline`). Removed keywords, AST variants, parser paths, analyzer
    arms, and tests. `docs/future-work.md` documents the runtime
    palette/nametable design for when it comes back.

Doc cleanup:

  - `docs/architecture.md` was describing files that don't exist
    (`analyzer/types.rs`, `optimizer/const_fold.rs`, `codegen/regalloc.rs`,
    `rom/header.rs`, `debug/symbols.rs`, …). Rewrote it to match the
    real flat `mod.rs` + `tests.rs` layout and the real pipeline order.
  - `docs/future-work.md` was a hybrid of open work and "recently
    completed" entries that duplicated the active stubs at the top of
    the file. Collapsed to just the gaps that are actually still open.
  - `README.md` claimed Mesen symbol export and 210 tests; updated both.
  - `docs/language-guide.md` and `spec.md` described `palette` decls,
    `set_palette` / `load_background`, `debug.overlay`, and error codes
    that were never emitted. Trimmed.
  - Stale comments on `Statement::Play`/`StartMusic`/`StopMusic`
    claimed the audio subsystem was "a no-op at codegen time".

Tests:

  - Regression tests for every fix above (`lower_shift_left_with_literal
    _count_uses_that_count`, `lower_shift_right_with_variable_count
    _uses_runtime_variant`, `lower_divide_emits_div_op_not_load_imm
    _zero`, `lower_modulo_emits_mod_op_not_load_imm_zero`,
    `strength_reduce_div_by_power_of_two`, `strength_reduce_mod_by
    _power_of_two`, `strength_reduce_shift_var_with_constant_amount`).
  - Renamed the `program_with_sprites_and_palette` integration test
    (which was exercising the now-removed `load_background`/`set_palette`)
    to `program_with_inline_sprite_chr`.

`examples/sprites_and_palettes.ne` lost its `palette`/`set_palette`
usage. Nothing in the emulator test presses A, so the headless
jsnes render shouldn't move, but the golden may need regeneration
via `UPDATE_GOLDENS=1` if it does.

https://claude.ai/code/session_012fKB251HvEUQwG3tizFyqt

docs/language-guide.md

@@ -641,22 +641,6 @@ Set the PPU scroll position:
 scroll(scroll_x, scroll_y)
 ```
 
-### Load Background
-
-Load a background nametable:
-
-```
-load_background TitleBG
-```
-
-### Set Palette
-
-Apply a palette:
-
-```
-set_palette GamePalette
-```
-
 ### Function Calls as Statements
 
 ```
@@ -680,29 +664,6 @@ sprite Coin {
 }
 ```
 
-### Palette Declarations
-
-Define color palettes using NES PPU color indices (`$00`-`$3F`):
-
-```
-palette GamePalette {
-    colors: [0x0F, 0x00, 0x10, 0x30,
-             0x0F, 0x07, 0x17, 0x27,
-             0x0F, 0x09, 0x19, 0x29,
-             0x0F, 0x01, 0x11, 0x21]
-}
-```
-
-Each group of 4 values is a sub-palette. The first color is typically `0x0F` (black) as the shared background color.
-
-### Background Declarations
-
-```
-background TitleBG {
-    chr: @chr("assets/title_screen.png")
-}
-```
-
 ### Asset Sources
 
 Three ways to provide asset data:
@@ -964,7 +925,6 @@ reference NEScript variables.
 | Code   | Description                |
 |--------|----------------------------|
 | E0201  | Type mismatch              |
-| E0202  | Invalid cast               |
 | E0203  | Invalid operation for type |
 
 ### Memory Errors (E03xx)
@@ -979,7 +939,6 @@ reference NEScript variables.
 |--------|----------------------------|
 | E0401  | Call depth exceeded        |
 | E0402  | Recursion detected         |
-| E0403  | Unreachable state          |
 | E0404  | Transition to undefined state |
 
 ### Declaration Errors (E05xx)
@@ -999,7 +958,7 @@ reference NEScript variables.
 | W0101  | Expensive multiply/divide operation      |
 | W0102  | Loop without break or wait_frame         |
 | W0103  | Unused variable                          |
-| W0104  | Unreachable code                         |
+| W0104  | Unreachable code (after return/break/transition, or state unreachable from start) |
 
 ### Example Error Output