Reduce allocations and redundant work across compiler hot paths - #21516

All changes are behavior-preserving and validated with isolated before/after
micro-benchmarks, profiled full-build A/B (GC-event count as a deterministic
allocation-churn proxy), full ConfigTestCases (8668, 548 snapshots),
TestCasesProduction, and yarn tsc.

Exports / reexport resolution (dominant cost on barrel-heavy graphs per CPU
profiling — ~21% fewer GC events on a reexport-heavy full build):
- ExportsInfo.findTarget: pass a shared empty visited-set (_findTarget only
  reads it) instead of a fresh Set per call.
- ExportsInfo.setTarget: copy exportName only when actually stored.
- isStarReexportBackToParent: allocate the visited set lazily.

ESM dependency templates (per import specifier / per reexport item):
- HarmonyImportSpecifierDependency: reuse the connection resolved in apply()
  instead of re-fetching it in _getCodeForIds; hoist the imported module's
  exports info out of the destructuring-replacement loop.
- HarmonyExportImportedSpecifierDependency: hoist getConnection and both
  getExportsInfo lookups out of the normal-reexport item loop; build
  ignoredExports in getStarReexports without the spread array.
- HarmonyImportDependency: drop a redundant template-literal re-wrap of
  userRequest.

Code generation:
- InitFragment.addToSource: flat index sort + numeric keyless keys.
- Template.getModulesArrayBounds: arithmetic module-id digit count, no string.
- HtmlGenerator: memoize module-by-identifier map per compilation (WeakMap).

Module concatenation (ConcatenatedModule):
- referencesByName single Map lookup; fold the build() dependency filter into
  the loop; build allIdentifiers Set without spreads.

Usage analysis (FlagDependencyUsagePlugin):
- Hoist the two stateless setUsedConditionally predicates; lazily allocate the
  usually-empty mangleableEscapeModules set.

Hashing (per module x runtime, every build):
- createModuleHashes: build the moduleHash cache key once per runtime.
- ChunkGraph graph-hash: hoist activeStateToString to module scope; compute
  strict only on cache miss.

Chunk splitting (SplitChunksPlugin):
- Remove the dead reusableChunks Set; hoist the loop-invariant
  getKey(usedChunks) out of the per-module loop.

Deno's node:fs.watch compatibility drops or delays change events, so the
WatchDetection cases intermittently time out waiting for the final rebuild.
Poll (only under Deno) for deterministic change pickup; Node and Bun keep
exercising native watching.