feat(chat): freeze QENC v2 private attachments

[QuickMythril]

Summary

• introduce strict QENC v2 framing for new private chat attachments while preserving QENC v1 and legacy resource compatibility
• add the Core reference codec for direct sender/recipient wrapping, QPGC-derived group attachment keys, and encrypted QATT filename/media-type/digest/file payloads
• commit independently generated language-neutral vectors, negative cases, sender-reopen coverage, and exact complete-envelope 1 MiB limits
• mark Core roadmap milestone C6 complete and record the corresponding Home desktop/Android implementation requirements

Security and compatibility

• direct attachments wrap to exactly both distinct CHAT participants in the direct-message profile
• group attachments bind the positive group id, full QPGC epoch id, and full QPGC key id, and derive a domain-separated content key instead of reusing the message key
• fixed and mode headers are authenticated; recipient ids are canonical, unique, and bounded; sensitive metadata stays inside the encrypted QATT payload
• Core continues recognizing historical QENC v1 and legacy prefixes without reinterpreting their bytes
• the existing QCHAT_ATTACHMENT_PRIVATE 1 MiB service limit counts the complete stored envelope

Verification

• focused exact-commit Maven gate: 67 tests passed
• full deterministic Maven suite: 3,175 tests passed, 0 failures, 0 errors, 67 skipped
• package build passed and produced the shaded Core JAR
• git diff --check passed

The first full-suite run encountered the pre-existing BlockArchiveTests.testServeArchivedBlocksForGetBlocksRequest synthetic null-mint flake. That exact test passed alone, and the subsequent complete 3,175-test rerun passed cleanly.

Follow-on

Home H6 can consume qenc-attachment-v2.json next to implement native source-token encryption, publish, authorized decrypt/view/save/stream, and immutable Chat descriptors without exposing keys to the app.