fix: interoperate with bounded age GREASE stanzas

[fujibee]

Summary

• revise the unpublished age-v1 profile to follow age unknown-stanza/GREASE interoperability
• preserve exact X25519 manifest-count enforcement and native-identity-only decryption
• bound header parsing to 256 X25519 / 512 total stanzas, 64 KiB header bytes, and 4096-byte lines
• add a real Rust age crate 0.12.1 GREASE fixture plus adversarial parser tests

Validation

• node --test tests/sync_cipher.test.mjs (8/8)
• bats tests/test_sync_cipher.bats (3/3)
• node docs/spec/vectors/verify-age-v1-vectors.mjs (19 vectors)
• node --test tests/remote_sync_engine.test.mjs (21/21)
• diff/syntax checks clean

Draft/unmerged dogfood stack. Merge remains koit-gated.

[fujibee]

Closing as fully absorbed, not abandoned. All four files this draft carries (docs/spec/age-v1-profile.md, docs/spec/vectors/age-v1-rust-grease.json, scripts/internal/sync-cipher.mjs, tests/sync_cipher.test.mjs) are already on the integration/remote branch with identical blob hashes as of head 189e551, so nothing remains here to merge.

The GREASE-tolerance change itself (accept bounded grease stanzas, reject active non-X25519 namespaces before the grease check) went through review and travels with the integration/remote train; it will reach main when that train lands as a whole.

Closing so the open-PR list reflects work that is actually pending.