M4+M5: Optimizer passes, type casting, bank switching, math runtime

Milestone 4 — Optimization & Polish:
- Strength reduction: multiply by power-of-2 → shift left
- Zero-page promotion analysis: rank variables by access frequency
- `as` type casting expression in parser/AST/analyzer
- `scroll(x, y)` statement
- `--asm-dump` flag for viewing generated assembly
- Extended optimizer tests (strength reduction, frequency analysis)

Milestone 5 — Bank Switching & Release:
- Mapper support: MMC1 (1), UxROM (2), MMC3 (4) in parser and ROM builder
- Bank declarations: `bank Name: prg` / `bank Name: chr`
- Linker::with_mapper for mapper-aware ROM generation
- Software multiply (8x8→16, shift-and-add algorithm)
- Software divide (8÷8→8, restoring division algorithm)
- ROM tests for mapper encoding round-trip
- Integration test for MMC1 compilation

210 tests total (18 new), all pre-commit checks pass.

https://claude.ai/code/session_01W6eQFStA66EuMKHUFo2rx3

src/rom/mod.rs

@@ -1,7 +1,7 @@
 #[cfg(test)]
 mod tests;
 
-use crate::parser::ast::Mirroring;
+use crate::parser::ast::{Mapper, Mirroring};
 
 /// iNES header magic bytes
 const INES_MAGIC: [u8; 4] = [0x4E, 0x45, 0x53, 0x1A]; // "NES\x1A"
@@ -137,3 +137,13 @@ pub struct RomInfo {
     pub mapper: u8,
     pub mirroring: Mirroring,
 }
+
+/// Map a `Mapper` enum variant to its iNES mapper number.
+pub fn mapper_number(mapper: Mapper) -> u8 {
+    match mapper {
+        Mapper::NROM => 0,
+        Mapper::MMC1 => 1,
+        Mapper::UxROM => 2,
+        Mapper::MMC3 => 4,
+    }
+}