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
• split verifier tests/text helper into submodules to satisfy the 1500-line LOC gate

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
• LOC gate: PASS (verify.rs 1489 lines)
• SDK/package version coupling: PASS
• release tag check for v3.10.0: PASS
• history policy gate: 0 new findings
• independent split review: 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.

Supersedes #1606 and #1607 without force-pushing.

[yvgude]

Superseded before merge: Delivery CI proved the signed OCLA contract pack must remain byte-stable. The unrelated portal-version edit is removed; the complete local delivery/security gate now passes. Clean replacement follows without force-push.