Generate handle table wasm functions in fused module

[avrabe]

Part of #69, depends on #70

Task

For each re-exporter, generate three wasm functions in the fused core module that operate on the handle table allocated in #70.

Implementation

Files: meld-core/src/merger.rs (or new meld-core/src/handle_table.rs)

Generate using wasm_encoder::Function + wasm_encoder::Instruction:

;; ht_new: store rep, return aligned handle
(func (param i32) (result i32)
  global.get $next_ptr
  local.tee $handle
  local.get 0        ;; rep
  i32.store           ;; store in memory N at handle addr
  local.get $handle
  i32.const 4
  i32.add
  global.set $next_ptr
  local.get $handle)

;; ht_rep: load rep from handle address
(func (param i32) (result i32)
  local.get 0
  i32.load)           ;; load from memory N

;; ht_drop: zero out entry
(func (param i32)
  local.get 0
  i32.const 0
  i32.store)          ;; store in memory N

Critical: i32.load/i32.store must specify the correct memory index for multi-memory mode.

Add new_func, rep_func, drop_func merged function indices to HandleTableInfo.

Branch

feat/per-component-handle-tables

Acceptance

cargo test --package meld-core passes. Functions generated but not yet wired.