chore(codegen-wasm): split crates/perry-codegen-wasm/src/emit.rs (10k lines, one mega WasmModuleEmitter impl)

[proggeramlug]

Part of #1097.

Current state

crates/perry-codegen-wasm/src/emit.rs is 9,988 lines with only 12 top-level functions — because almost everything is methods on the WasmModuleEmitter struct (one giant impl block starting at line 616).

The two methods that dominate the file are:

• emit_stmt — lines 4547 → 4956 (~410 lines — manageable)
• emit_expr — lines 4956 → 9544 (~4.6k)
• emit_js_stmt — lines 2413 → 2579 (handles JS-fallback emission, used when WASM can't represent something)
• emit_js_expr — lines 2579 → 3035 (the JS-fallback expression emitter)
• collect_strings + collect_strings_in_stmts + collect_strings_in_stmt + collect_strings_in_expr — 3035 → 3993 (~1k)
• emit_method_call — 4203 → 4529

Proposed split

Move emit.rs to crates/perry-codegen-wasm/src/emit/:

• emit/mod.rs — compile_to_wasm, compile_to_wasm_with_async, WasmModuleEmitter::new, intern_string, get_type_idx, compile, the core type/field/function-index machinery (~533–2175), plus the small free-functions f64_const, f64_const_bits, map_ui_method
• emit/function.rs — compile_function, compile_closure, compile_class_constructor, compile_class_method, emit_frame_begin, emit_slot_addr, emit_store_arg, emit_store_const, emit_local_or_global_get (~2175–2378, ~4023–4114)
• emit/string_collection.rs — collect_strings, collect_strings_in_stmts, collect_strings_in_stmt, collect_strings_in_expr (~3035–3993)
• emit/memcall.rs — emit_memcall, emit_memcall_void, emit_memcall_i32, RuntimeImports::rt access helper (~4114–4203)
• emit/method_call.rs — emit_method_call (~4203–4529)
• emit/binary.rs — emit_bitwise_binary
• emit/stmt.rs — emit_stmt, has_return, expr_has_value (~4547–4956, ~9544+, ~9956+)
• emit/expr.rs — emit_expr — the 4.6k beast. Once isolated, the natural sub-split is by Expr::* variant the same way chore(codegen): split crates/perry-codegen/src/lower_call.rs (11.8k lines, lower_call + extract_options_fields) #1099 splits the LLVM expr.rs — but suggest landing the file move first and the within-emit_expr split as a follow-up so reviewers can stage the work.
• emit/js_fallback.rs — emit_js_async_function, emit_js_stmt, emit_js_expr (~2378–3035)
• emit/closures.rs — collect_closures_from_stmts, collect_closures_from_expr (~9699–9956)
• emit/locals.rs — collect_locals, collect_module_let_ids, resolve_source_module_idx (~9578–9699)

Hot path caveat

WASM codegen issues are tracked in #1081 right now. Hold this refactor until the active stack-balance bug is resolved or coordinate with whoever is currently in this file — don't rebase a 10k-line move on top of in-flight fixes.

Acceptance

Same conventions as #1097. Plus: smoke-test by compiling at least one TypeScript program to --backend wasm before and after.