Sprite resolution, asset wiring, shift-assign, unreachable state warning

Sprite/asset pipeline:
- Linker::link_with_assets() places sprite CHR data in ROM at correct tile
- assets::resolve_sprites() walks Program for inline sprite bytes
- CodeGen::with_sprites() maps sprite names to tile indices
- gen_draw() uses correct tile index from sprite declarations
- main.rs wires the full resolution pipeline

Shift-assign operators (<<= and >>=):
- AssignOp::ShiftLeftAssign and ShiftRightAssign variants
- Parser handles in both statement and array index contexts
- Codegen emits ASL A / LSR A
- IR lowering maps to ShiftLeft/ShiftRight ops

Unreachable state warning (W0104):
- BFS from start state finds reachable states via transitions
- States not reached produce W0104 warning

Error polish helpers:
- suggest_var_name() for "did you mean" suggestions
- emit_undefined_var() for E0502 with typo hints
- Used by analyzer for better diagnostics

242 tests pass, clippy clean.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/assets/resolve.rs

@@ -0,0 +1,55 @@
+use std::path::Path;
+
+use crate::linker::SpriteData;
+use crate::parser::ast::{AssetSource, Program};
+
+/// Resolve sprite declarations in a program into concrete CHR byte blobs and
+/// assign each one a tile index in CHR ROM.
+///
+/// Tile index 0 is reserved for the built-in default smiley sprite, so user
+/// sprites start at tile index 1. A single sprite declaration may occupy
+/// multiple consecutive tiles if its CHR data is larger than 16 bytes.
+///
+/// `source_dir` is used as the base for `@binary` / `@chr` relative paths.
+/// For now, only [`AssetSource::Inline`] sources produce sprite data; file-
+/// backed sources are parsed but skipped here (future work), which keeps
+/// existing tests that reference missing files compiling without I/O errors.
+pub fn resolve_sprites(program: &Program, source_dir: &Path) -> Result<Vec<SpriteData>, String> {
+    let _ = source_dir; // reserved for future file-backed resolution
+    let mut sprites = Vec::new();
+    // Tile index 0 is the built-in smiley; user sprites start at 1.
+    let mut next_tile: u8 = 1;
+
+    for sprite_decl in &program.sprites {
+        let chr_bytes = match &sprite_decl.chr_source {
+            AssetSource::Inline(bytes) => bytes.clone(),
+            // Binary/Chr loading from files is future work; skip for now so
+            // programs that reference (possibly-missing) external assets
+            // still compile without I/O errors.
+            AssetSource::Binary(_) | AssetSource::Chr(_) => continue,
+        };
+
+        // Each NES 8x8 tile is 16 bytes of 2-bitplane CHR data. A single
+        // sprite declaration can span multiple tiles when its CHR blob is
+        // longer than 16 bytes.
+        let tile_count = chr_bytes.len().div_ceil(16);
+        if tile_count == 0 {
+            continue;
+        }
+        if next_tile as usize + tile_count > 256 {
+            return Err(format!(
+                "sprite '{}' would exceed CHR ROM tile limit",
+                sprite_decl.name
+            ));
+        }
+
+        sprites.push(SpriteData {
+            name: sprite_decl.name.clone(),
+            tile_index: next_tile,
+            chr_bytes,
+        });
+        next_tile += tile_count as u8;
+    }
+
+    Ok(sprites)
+}