Allocate handle table space in merger

[avrabe]

Part of #69

Task

For each re-exporter component (identified via graph.reexporter_components), allocate a region in its linear memory for a handle table.

Implementation

Files: meld-core/src/merger.rs

1. Add HandleTableInfo struct:

pub struct HandleTableInfo {
    pub memory_idx: u32,
    pub next_ptr_global: u32,
    pub table_base_addr: u32,
    pub capacity: u32,
}

2. Add pub handle_tables: HashMap<usize, HandleTableInfo> to MergedModule

3. After merging all components, for each re-exporter:

   • Find memory index from memory_index_map[(comp_idx, 0, 0)]
   • Scan data segments to find highest offset + data.len() for that memory
   • Round up to 4-byte alignment → table_base_addr
   • Add mutable i32 global initialized to table_base_addr + 4 (skip slot 0)
   • Ensure memory has enough pages for table (256 entries = 1024 bytes)

Branch

feat/per-component-handle-tables

Acceptance

cargo test --package meld-core passes (73 tests, no regression).