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/linker/mod.rs

@@ -3,13 +3,14 @@ mod tests;
 
 use crate::asm;
 use crate::asm::{AddressingMode as AM, Instruction, Opcode::*};
-use crate::parser::ast::Mirroring;
+use crate::parser::ast::{Mapper, Mirroring};
 use crate::rom::RomBuilder;
 use crate::runtime;
 
 /// Link compiled code into a complete NES ROM.
 pub struct Linker {
     mirroring: Mirroring,
+    mapper: Mapper,
 }
 
 /// A smiley face CHR tile for the default sprite (M1).
@@ -50,7 +51,14 @@ const DEFAULT_PALETTE: [u8; 32] = [
 
 impl Linker {
     pub fn new(mirroring: Mirroring) -> Self {
-        Self { mirroring }
+        Self {
+            mirroring,
+            mapper: Mapper::NROM,
+        }
+    }
+
+    pub fn with_mapper(mirroring: Mirroring, mapper: Mapper) -> Self {
+        Self { mirroring, mapper }
     }
 
     /// Link all code sections into a .nes ROM.
@@ -110,6 +118,7 @@ impl Linker {
 
         // Build ROM
         let mut builder = RomBuilder::new(self.mirroring);
+        builder.set_mapper(crate::rom::mapper_number(self.mapper));
         builder.set_prg(prg);
 
         // CHR ROM with default sprite tile