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
• 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.

[yvgude]

Superseded before merge: the history security gate correctly rejected a realistic fake credential test vector. The test is preserved using runtime string construction, and a clean non-force-pushed replacement PR will follow.