perf(codegen): FxHashMap cascade pt2 — value_sink/const_fold/fusion/register_estimate (+57.6%)

[TheTom]

Summary

Continues #235 (licm) and #236 (copy_prop/alg_simplify/unroll/vectorize) into the four remaining codegen passes that still used BTree containers keyed by `ValueId` (or `usize` position) for pure get/insert/contains:

File	Change
`value_sink.rs`	`compute_global_use_count`'s use-count map (`BTreeMap<ValueId, usize>` → `FxHashMap`). `rebuild_with_sinking`'s `remove_at` set + `insert_before` map (BTreeSet/BTreeMap of `usize` → Fx). All pre-sized from outer plans/op counts.
`const_fold.rs`	DCE's `used` set + cross-block `cross_block_refs` (`BTreeSet` → `FxHashSet`). `collect_uses` helper signature updated.
`fusion.rs`	`pinned_per_block` inner value (`BTreeSet` → `FxHashSet`); outer was already `FxHashMap`. `fuse_block` only does `.contains()` on the inner set — no iteration order.
`register_estimate.rs`	`block_max_live`'s `live` set (`BTreeSet` → `FxHashSet`), pre-sized to `block.ops.len()`.

`type_check.rs` intentionally NOT touched — overlaps with your in-flight PR #58 scope (introduces a new typed-value-environment model).

Measured (M5 Max, release, 2000-op block, 10K iters side-by-side)

`perf_block_max_live_btree_vs_fxhash` — BTreeSet impl kept inline in the test module:

```
=== block_max_live: 2000-op live-set walk × 10000 iters ===
BTreeSet (old): 40110.2 ns/call
FxHashSet (new): 17000.2 ns/call
→ speedup : 2.36× (+57.6%)
```

Pinned by `assert!(fx_ns_per * 1.05 <= bt_ns_per)`.

`cargo test -p metaltile-codegen --release perf_block_max_live_btree_vs_fxhash -- --ignored --nocapture`

Test plan

• `cargo test -p metaltile-codegen --lib` — 161/161 pass
• `cargo clippy --workspace --all-targets --all-features -- -D warnings` — clean
• `cargo fmt --all --check` — clean
• `cargo test -p metaltile-codegen --release perf_block_max_live_btree_vs_fxhash -- --ignored --nocapture` — +57.6%