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

tests/integration_test.rs

@@ -802,7 +802,7 @@ fn audio_pipeline_drops_period_table_cost_when_unused() {
 // ── M3 Tests ──
 
 #[test]
-fn program_with_sprites_and_palette() {
+fn program_with_inline_sprite_chr() {
     let source = r#"
         game "M3 Assets" { mapper: NROM }
 
@@ -811,22 +811,10 @@ fn program_with_sprites_and_palette() {
                   0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00, 0x00]
         }
 
-        palette MainPal {
-            colors: [0x0F, 0x00, 0x10, 0x20]
-        }
-
-        background TitleBg {
-            chr: @binary("title.bin")
-        }
-
         var px: u8 = 128
         var py: u8 = 120
 
         state Title {
-            on enter {
-                load_background TitleBg
-                set_palette MainPal
-            }
             on frame {
                 if button.right { px += 2 }
                 if button.left  { px -= 2 }