cleanup(codegen): remove dead Codegen fields region_stack + bump_ptr (+ orphaned RegionInfo) (#250)

## Summary

Per typed-wasm proposal 0001 Appendix B "Producer-readiness checklist"
follow-up. Both fields in \`Codegen\` were marked
\`#[allow(dead_code)]\`
with the comment "reserved for future interpreter use" — declared,
initialized in \`Codegen::new()\`, never read or written elsewhere in
the crate.

## Removed

- \`Codegen.region_stack: Vec<RegionInfo>\` field + its constructor init
- \`Codegen.bump_ptr: u32\` field + its constructor init (adjacent dead
  code in the same struct, same "reserved" comment)
- \`RegionInfo\` struct (only consumer was \`region_stack\`; orphaned
  after removal — its own fields were already \`#[allow(dead_code)]\`)

## Retained

- \`REGION_HEADER_SIZE\` constant — \`pub\`, conceptually describes the
  linear-memory layout (used by comments at lines 28 + 76 + within the
  wasm-side memory-layout code at ~1506-1531 that manipulates the
  wasm-memory \`bump_ptr\` cell at offset 0). Removing the Rust-side
  dead struct field does not affect the wasm-side memory layout.

## Important disambiguation

The wasm-side \`bump_ptr\` (a value stored in wasm linear memory at
offset 0, real and load-bearing) is unrelated to the removed Rust
\`Codegen.bump_ptr\` struct field of the same name. The struct field
was a stale carry-over from an earlier design where the Codegen
tracked it Rust-side; the actual implementation tracks it inside the
emitted wasm. The naming collision is what kept the dead field
camouflaged.

## Verification

- \`cargo check -p ephapax-wasm\` — clean, no warnings
- \`cargo test -p ephapax-wasm\` — 84/84 pass

## Carry-forward note

This closes the dead-code item from typed-wasm proposal 0001 Appendix B.
The access-sites codegen follow-up is filed at the ephapax counterpart
to affinescript#462.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Author: hyperpolymath

src/ephapax-wasm/src/lib.rs

@@ -263,12 +263,6 @@ const CLOSURE_SIZE: u32 = 8; // 2 x i32
 
 /// Code generator state.
 pub struct Codegen {
-    /// Current bump pointer for allocations (reserved for future interpreter use)
-    #[allow(dead_code)]
-    bump_ptr: u32,
-    /// Region stack for tracking active regions (reserved for future interpreter use)
-    #[allow(dead_code)]
-    region_stack: Vec<RegionInfo>,
     /// Generated WASM module
     module: WasmModule,
 
@@ -387,16 +381,6 @@ struct LambdaInfo {
     body: Expr,
 }
 
-#[derive(Debug, Clone)]
-struct RegionInfo {
-    /// Region name
-    #[allow(dead_code)]
-    name: String,
-    /// Start of region allocations
-    #[allow(dead_code)]
-    start_ptr: u32,
-}
-
 impl Default for Codegen {
     fn default() -> Self {
         Self::new()
@@ -407,8 +391,6 @@ impl Codegen {
     /// Create a new code generator
     pub fn new() -> Self {
         Self {
-            bump_ptr: REGION_HEADER_SIZE,
-            region_stack: Vec::new(),
             module: WasmModule::new(),
             locals: LocalTracker::default(),
             data_entries: Vec::new(),