Add post-quantum (age ML-KEM-768 / X-Wing) scaffold for WebCrypt/onlyagent

[0c-coder]

Host-side post-quantum KEM scaffold for the WebCrypt/onlyagent app, mirroring the age PQC support in python-onlykey#90 + firmware libraries#29 (ML-KEM-768 = keytype 5, X-Wing = keytype 6).

New files only, no edits to existing files:

• src/onlykey-fido2/onlykey/xwing.js -- ML-KEM-768 / X-Wing encapsulation + age stanza helpers (host side)
• • src/onlykey-fido2/onlykey/onlykey-pqc.js -- device getPubKey / decapsulate over the existing OKCONNECT derive flow (DERIVE_PUBLIC_KEY / DERIVE_SHARED_SECRET), keyed by derivation label (no slots)
• • src/plugins/age/age-pqc.js -- age encrypt/decrypt plugin
• • src/plugins/age/INTEGRATION.md -- wiring (plugin.js DI, package.json deps), keytype map, and remaining TODO(verify) items
    Design: the web path has no key slots -- keys are derived per-identity from the reserved web-derivation key. The 32-byte derived secret feeds X-Wing directly (its private key is a 32-byte seed) and expands to ML-KEM's (d,z). Only the keytype byte (5/6) is added to the derive request.

Compatibility with #38 (OnlyAgent reskin): disjoint file sets. #38 only touches src/app-src.html, src/index-src.html, and two asset files; this only adds device-layer JS and a new src/plugins/age/ folder. They merge cleanly in either order, and the reskin's app_pages loop auto-styles the new age plugin page.

Status: scaffold / WIP. Encapsulation and the derive-flow shape are real; the TODO(verify) items in INTEGRATION.md (identity->keyhandle encoding, decaps op, ML-KEM seed expansion, age stanza/HPKE) must be matched byte-for-byte against python-onlykey#90 (tests/test_age_wire.py) before interop.

[0c-coder]

Related post-quantum (ML-KEM-768 / X-Wing) work across the stack:

• Firmware: Restrict host key-writes to user ECC slots (block reserved 117-132, keep backup) trustcrypto/libraries#29 — restrict host key-writes to user ECC slots; PQ seeds live in 101–116.
• CLI / age plugin: age plugin: align PQ host protocol with firmware; 1.2.11 packaging/description trustcrypto/python-onlykey#90 — host protocol aligned with firmware (slots, key type, OKSETPRIV keygen, multi-packet decaps).
• Agent packaging: onlykey-agent: add PyPI project description and bump to 1.1.16 lib-agent#18 — onlykey-agent 1.1.16.