feat(ctxpkg): release checkpoint layer in v3.10.0

[yvgude]

Summary

• add the fail-closed signed .ctxpkg v2 checkpoint layer
• align bounds and canonical Engine bindings with the SDK contract
• release the new backward-compatible public mechanism as Engine 3.10.0

Scope

Mechanism only: no SDK product lifecycle, commercial policy, Cloud, or private research code.

Verification

• focused core::context_package: 120 passed
• cargo clippy --all-features -- -D warnings: PASS
• cargo fmt --check: PASS
• SDK/package version coupling: PASS
• release tag check for v3.10.0: PASS
• history policy gate: 0 new findings
• prior full local library run: 10,487 passed, 22 ignored, one /private/tmp path-jail artifact; the same test passes from the normal project path, so protected PR CI is authoritative

SemVer

Minor bump: this is a new backward-compatible public package mechanism.

Supersedes #1606 without force-pushing; the realistic fake credential test vector is now constructed only at runtime.

[yvgude]

Superseded before merge: CI exposed the 1500-line LOC gate. The verifier has now been mechanically split into submodules; focused tests, clippy, fmt, LOC, and history-policy gates pass. A clean non-force-pushed replacement follows.