P2 E3: clear parser-owned Ed25519 signing-key bytes after configuration load

[gnanirahulnutakki]

Problem

loadSessionPrivateKey clears the mounted PEM bytes and its caller clears the returned key, but Go PKCS8 parsing creates a separate Ed25519 private-key allocation. The loader copies that parsed key for ownership transfer and leaves the parser-owned bytes for garbage collection.

Parent: #21.

Required fix

• Copy the validated parsed Ed25519 private key into the returned owned slice.
• Clear the parser-owned private-key slice before returning.
• Preserve existing fail-closed PEM, PKCS8, key type, key length, read-only regular-file, size, and public/private match validation.
• Add a focused regression that proves the returned copy remains usable and independent after the internal source is cleared.

Acceptance criteria

• The parser-owned Ed25519 private-key bytes are explicitly cleared on the success path.
• The returned key is a distinct full-length copy and remains cryptographically usable.
• Error behavior and deployment inputs remain unchanged.
• Focused race tests and repository quality/security gates remain green.

Security and cost

This narrows the in-process lifetime of one redundant signing-key copy. Go does not guarantee complete secret erasure across compiler/runtime copies, so this is defense in depth rather than a memory-forensics guarantee. It adds no network, cloud, storage, or recurring cost.

Primary sources

• Go crypto/x509 PKCS8 parser: https://pkg.go.dev/crypto/x509#ParsePKCS8PrivateKey
• Go Ed25519 seed expansion: https://pkg.go.dev/crypto/ed25519#NewKeyFromSeed

[gnanirahulnutakki]

Landed and verified through PR #313.

Immutable closure proof:

• exact GitHub-verified signed DCO/GSTACK feature commit: f3eac6b86f402495f5f31b478d41fa1761f979ee
• merge commit: e52e71a5e71a15bfeab3949184abc590d77b8bf2
• merge parents: prior dev fdf932bbaaebb61f3d3a999b1d07160b02087c74 and exact feature head f3eac6b86f402495f5f31b478d41fa1761f979ee
• exact post-merge dev CI: https://github.com/ArdurAI/sith/actions/runs/29975944998
• exact post-merge CodeQL: https://github.com/ArdurAI/sith/actions/runs/29975944930
• complete two-file local CodeRabbit review: zero findings; the reviewed file context was unchanged by the rebase
• focused race regression: 100 consecutive passes
• unresolved review threads: 0
• open security queues: 0 Dependabot / 0 code scanning / 0 secret scanning

The runtime now copies the validated Ed25519 key into owned memory and clears the parser-owned slice. Existing PEM, PKCS8, Ed25519, length, file-permission, size, and public/private-match validation remains fail-closed. This is defense in depth, not a guarantee that every compiler/runtime copy is erased, and it adds no network, cloud, storage, or recurring cost.