Add tamper-evident audit log (audit package, logx verify/keygen)

[AyoubTadlaoui]

Adds an audit subpackage: an append-only, hash-chained, optionally Ed25519-signed slog.Handler for tamper-evident logging, plus logx verify and logx keygen to check and key it.

What

• audit.NewHandler writes each record as a hash-chained entry. hash = SHA-256(seq, prev, time, data), with each field length-prefixed so crafted input cannot shift the boundaries. Editing, deleting, reordering, or inserting a line breaks the chain at that entry.
• With an Ed25519 signer, every entry is also signed, so entries cannot be re-forged into a clean chain without the private key.
• audit.Verify / VerifyFile walk a log offline and report the first broken entry. audit.OpenFile resumes an existing chain across restarts.
• logx verify [-pubkey k] file... checks integrity (exit 0 intact, 1 tampered, 2 unreadable). logx keygen writes an Ed25519 keypair. Existing logx pretty-print behavior is unchanged; the verbs dispatch only when they are the first argument.

Threat model

Detected: edits, deletes, reorders, insertions; with -pubkey, forgery or re-signing without the key.

Not detected on its own (documented in the package and README): dropping entries from the very end of the file, and a full rewrite of an unsigned log. Sign the log and keep the private key off the writing host; anchor the head hash externally if end-truncation is in scope.

Notes

• Zero external dependencies. Standard library only (crypto/sha256, crypto/ed25519, crypto/rand, encoding/pem). No crypto/x509.
• 106 test functions across the module pass under -race. gofmt -s, go vet, and golangci-lint are clean. Runnable example in examples/audit.