feat(supervisor): add first-class pool views with Option-returning get

[slepp]

Why

Supervisor pool accessors were fail-closed in two places: whole-field access on a ChildKind::Pool field (let workers = sup.workers) was rejected at HIR with SupervisorPoolChildAccessorUnsupported, and sup.pool.get(i) had no Option-returning form — only the trapping pool[i]. Both gaps were deferred from the earlier static-child accessor work; this completes the surface.

What

Pool accessors are now complete end-to-end via one substrate change — a first-class BuiltinType::SupervisorPool view type { LocalPid<S>, i64 pool_key }. The (supervisor, pool_key) pair travels as a typed runtime value, so len(), trapping [i], and get(i) all work on a bound view, not just the syntactic sup.pool.… form.

• types/checker: Pool-kind field access types as SupervisorPool<S,T>; get(i) types as Option<LocalPid<T>>; pool indexed assignment and unknown pool methods are rejected with typed errors
• HIR: the bare-pool fail-closed gate is deleted end-to-end (diagnostic variant, gate pass, hew-wasm mapping) — the construct is implemented, no compat shim
• MIR: the former NotYetImplemented Pool arm emits RecordInit{ sup_handle, pool_key }; get routes the existing hew_supervisor_pool_child_get aggregate-return ABI through a hew_supervisor_pool_get_option terminator; the old span-rekeyed PoolAccessor side table shrinks to {kind}
• codegen: lower_supervisor_pool_get_option_call materialises the registered tagged Option layout (Some=0 / None=1) via the shared enum-variant emission shell, mirroring the string/vec get-option precedent; all new error paths fail closed
• ownership: LocalPid is a borrow handle (supervisor owns the actor) — no retain/release, same contract as the shipped static-child accessor

Test

• supervisor_pool_get_option.hew fixture: exact stdout 84, proves Some-liveness via await worker.id() and None at both boundaries (get(-1), get(count))
• supervisor_pool_field_access.hew fixture: exact stdout 37, whole-field pool view bound and used
• hew-codegen-rs emission test pins store i8 0/store i8 1 for both Option arms and the payload-then-tag ordering
• hew-mir actor tests assert RecordInit(SupervisorPool), the hew_supervisor_pool_child_get call, and the get-option terminator are all reached
• hew-types adds pool_field_view_preserves_accessor_types_after_binding
• Full workspace nextest green; fixture ratchet clean

Quality Checklist

• PR title, body, and commit messages avoid internal-only orchestration/model vocabulary
• No new .ok()? or unwrap_or_default() in codegen without // JUSTIFIED comment
• New allocations have cleanup paths for sync, async, and actor shutdown contexts (no new allocations — the Some payload is a borrowed handle)
• Serialization changes include round-trip encode/decode tests (no serialization changes)
• New runtime features have WASM implementation or // WASM-TODO(#NNN): marker, and new hew_* exports are classified in scripts/jit-symbol-classification.toml (no new runtime exports — consumes the existing hew_supervisor_pool_child_get)
• No duplicated logic — checked for existing helpers before adding new ones (Option construction reuses the shared enum-variant emission shell)

Out of scope

• Restoring the multi-static-children compile test narrowed during the gate-test rewrite, and negative checker tests for pool indexed assignment / unknown pool methods — small follow-ups
• Deduplicating the two hand-built {ptr, i64} SupervisorPool struct layouts (resolve_ty / record_struct_for) into a shared helper
• The standing stale-handle-across-yield caveat on borrow handles is a documented substrate property, unchanged here