Cache the in-flight xxhash-wasm initialization so concurrent calls reuse
the same promise before the raw hash function is available.
This avoids redundant loader work and prevents races during early CCH
computations.
Squash merge of feat/claude-code-native-parity into release/v3.6.5.
## Wiring
1. base.ts: CCH xxHash64 body signing for anthropic-compatible-cc-* providers,
applied after CLI fingerprint ordering so the hash covers the final bytes
sent upstream.
2. chatCore.ts: After buildClaudeCodeCompatibleRequest(), applies synchronous
parity pipeline steps:
- remapToolNamesInRequest() — TitleCase tool name mapping (14 tools)
- enforceThinkingTemperature() — temperature=1 when thinking active
- disableThinkingIfToolChoiceForced() — remove thinking on forced tool_choice
Cache-control limit enforcement intentionally omitted from chatCore because
the billing-header system block counts toward the 4-block cap and would strip
legitimate client cache markers.
3. xxhash-wasm: installed (was declared in package.json by PR but not installed).
## Test updates
- tests/unit/claude-code-parity.test.mjs: 25 new tests covering CCH signing,
fingerprint computation, tool remapping, and API constraints.
- tests/unit/cc-compatible-provider.test.mjs: fixed pre-existing assertion that
expected no cache_control on system blocks (billing header now carries ephemeral
per PR #1188 design).
Closes#1188
