Each distinct `FxHashMap<K, V>` monomorphizes its own copy of hashbrown; maps keyed by the dense u32 newtype ids become direct-indexed `IndexVec<Id, Option<V>>`s, removing those instantiations and the per-lookup hashing.

### Migration criteria

A map was migrated only when all of these hold:

1. The key is a dense id (`IdentifierId`, `DeclarationId`, `BlockId`) with a sound bound available at construction time: `env.identifiers.len()` (declaration ids share the identifier id index space — `DeclarationId::from_usize(id.index())` at allocation), `env.next_block_id_counter` (+1 for the synthetic exit node in the dominator), or the max block id present in the function body.
2. The map is only accessed by key (`get`/`insert`/`contains_key`); it is never iterated in a way that can reach the output. The one iterated map (the optional-chain temporaries merge) writes into another map with distinct keys, so iteration order is not observable.
3. `Option<V>` slots preserve the exact presence semantics, including "insert twice keeps latest" and `entry().or_insert()` first-wins. Where `map.len()` carried meaning, it was replaced with an explicit counter (`Rewrites::len` for the phi-elimination fixpoint, `BlockIdMappings::next` for sequential id assignment).
4. The map is allocated once per pass, not per inner loop iteration.

### Migrated maps

| File | Map |
| --- | --- |
| `react_compiler_hir/dominator.rs` | `PostDominator::nodes`, `Graph::node_index`, `raw_nodes`, `doms` (kills `FxHashMap<BlockId, BlockId>`, `<BlockId, Node>`, `<BlockId, usize>`) |
| `react_compiler_reactive_scopes/stabilize_block_ids.rs` | block id `mappings` (new `BlockIdMappings` with explicit `next` counter) |
| `react_compiler_optimization/prune_maybe_throws.rs` | `terminal_mapping` |
| `react_compiler_ssa/eliminate_redundant_phi.rs` | `rewrites` (new `Rewrites` with explicit entry count for the fixpoint check) |
| `react_compiler_validation/validate_hooks_usage.rs` | `value_kinds` (`Option<Kind>` is 1 byte) |
| `react_compiler_reactive_scopes/promote_used_temporaries.rs` | `State::pruned`, `inter_state` |
| `react_compiler_reactive_scopes/merge_reactive_scopes_that_invalidate_together.rs` | `last_usage`, `temporaries` |
| `react_compiler_inference/propagate_scope_dependencies_hir.rs` | `temporaries` + `temporaries_read_in_optional` (the crate's most-instantiated map type, ×11), the declaring-scope map in `find_temporaries_used_outside_declaring_scope`, and `DependencyCollectionContext::{declarations, reassignments}` |

This eliminates 11 distinct id-keyed hashbrown instantiations from the crate.

### Deliberately skipped

- `outline_jsx.rs` `globals` and `infer_types.rs` / `name_anonymous_functions.rs` `names`: these create a fresh map per nested function, so a dense vec sized by the env-wide identifier count would allocate O(identifiers) for a handful of entries on every nested function.
- `outline_jsx.rs` `rewrite_instr` and other `EvaluationOrder`-keyed maps: no clean bound at the construction site and keys are sparse.
- `get_assumed_invoked_functions` temporaries in `propagate_scope_dependencies_hir.rs`: allocated per inner function (same per-iteration concern).
- `FxIndexMap` uses: insertion order is semantic there.
- Maps whose iteration feeds diagnostics or output ordering (e.g. `validate_no_derived_computations_in_effects.rs`).

### Measured effect

Release example binary (`examples/react_compiler`, macOS arm64): `__text` (machine code) shrinks 2,767,800 → 2,756,116 bytes (**−11,684 bytes, −0.42%**). The on-disk file size only changes by 16 bytes because the `__TEXT` segment is page-aligned; the machine-code section is the meaningful metric. The example links the full parser/codegen stack, so the relative saving within this crate's own code is larger. The main win is runtime: every migrated lookup is now a bounds-checked array index instead of a hash + probe.

The upstream fixture corpus (1,736 snapshots) is byte-for-byte unchanged, and `cargo clippy -p oxc_react_compiler --all-targets` is clean.

### Note for upstream re-syncs

This intentionally deviates from the upstream TypeScript structure (`Map`-based sidemaps in `PropagateScopeDependenciesHIR.ts`, `Dominator.ts`, etc.). When re-syncing this crate with upstream via `react_compiler_oxc`, please preserve the `IndexVec` representation for these maps.

Authored with Claude Code; reviewed by a human before merge.