Context

Phase 1 of #573 migrated Map<K, V> to the heap-wrap-as-ADT scheme. This issue tracks the parallel migration for Decimal, which still uses raw i32 handles into _decimal_store and therefore still leaks per execute().

Decimal differs slightly from Map and Set: there are many more host helpers (decimal_from_int, decimal_from_string, decimal_from_float, decimal_add, decimal_sub, decimal_mul, decimal_div, decimal_round, ...) and most return new handles, so the wrap migration touches every arithmetic call-site.

Scope

1. vera/wasm/calls_containers.py — wrap on every Decimal-returning op (≈10 sites covering construction + binary ops + round); unwrap on every Decimal-consuming op (decimal_eq, decimal_compare, decimal_to_string, decimal_to_float).
2. vera/wasm/helpers.py — remove "Decimal" from _HOST_HANDLE_TYPES.
3. vera/codegen/api.py::host_decref_handle — add kind == 3 branch popping _decimal_store.
4. vera/browser/runtime.mjs::host_decref_handle — same kind === 3 branch popping decimalStore.
5. Define _WRAP_KIND_DECIMAL = 3 and _DECIMAL_HANDLE_TAG = 0xFEEDC003 constants alongside the Map equivalents (already reserved in the phase-1 PR).
6. Regression test for a heavy Decimal arithmetic loop showing _decimal_store stays bounded.
7. Update TestOpaqueHandleParamRooting347::test_decimal_param_not_shadow_pushed for the post-migration expectation.

Acceptance

• 5 000-iteration decimal_add chain: _decimal_store size at exit < 100.
• All existing Decimal tests still pass.
• Browser parity tests pass.
• Decimal is removed from _HOST_HANDLE_TYPES.

Notes for the implementor

• array_fold over Decimal accumulators is the natural stress shape (mirrors TestArrayFoldHandleRooting490 with the post-migration expectation flipped — accumulators ARE rooted now).
• Watch for decimal_round which has a different signature than the other binary ops (takes a precision arg as i64).