[codex] Add WASM Chrome extension PoC

[michalpalkowski]

Summary

• make Rayon optional behind parallel-rayon and add scalar fallbacks for the signing/tree hot paths
• add a wasm-bindgen wrapper crate with demo and 2^18 / w=2 production-benchmark feature modes
• add an MV3 Chrome extension PoC with keygen, sign, detached verify, preparation advance, and in-extension perf sampling
• document local run commands for demo vs production builds

How to run

./examples/chrome-extension-poc/build.sh
# or, for larger benchmark-oriented params:
./examples/chrome-extension-poc/build.sh production

Then open chrome://extensions, enable Developer mode, load examples/chrome-extension-poc, and use Generate Demo Key, Sign, Verify, and Run Perf Sample.

Validation

• cargo check -q --no-default-features
• cargo check -q -p leansig-wasm --target wasm32-unknown-unknown --features production-lifetime-18-w2
• /bin/bash examples/chrome-extension-poc/build.sh production

Notes

The extension verifier is a WASM/JS-side detached verifier: it verifies from public key bytes, epoch, message bytes, and signature bytes without secret-key access. This PR does not add an on-chain verifier.