feat: persist + expose proven kernel signing address at registration (evidence #52)

[LamaSu]

Why

The oracle authenticates kernel-signed machine logs (evidence primitive #52, machine.execution_log settlement) against a kernel's real secp256k1 signing address. Today registration threw that address away and stored a random value (kernel.facade.ts:313 publicKey: 0x${randomBytes(32)}), so the oracle had nothing to verify against — the root blocker of the machine-log money path.

What

Persist the kernel's REAL signing address at registration, with proof-of-possession, and expose it on the kernel DTO.

• DB (@pcc/store): new nullable signing_address column on shop_kernels (drizzle schema + idempotent safeAddColumn for existing DBs). $inferInsert auto-derives — no repo change.
• Gateway (@pcc/gateway) register(): recovers the signer from signingProof (viem recoverMessageAddress over a kernelId-bound message) and persists signingAddress only if it matches the claimed address. Absent / mismatched / malformed / replayed proof → null. Fail closed — never persist an unproven signer (a wrong one could clear settlement and release escrow).
• Set-once hardening: a generic upsert never overwrites an already-proven signer (key rotation is a dedicated owner-authenticated op — follow-up).
• DTO: KernelDTO.signingAddress: string | null (checksummed when proven, else null) via the populator → flows to GET /api/kernels/:id (oracle fetch) and GET /api/kernels.
• Kernel (@pcc/kernel): KernelKeychain.buildRegistrationProof(kernelId) returns { signingAddress, signingProof }; kernelSigningProofMessage() is the single source of truth for the message format, imported by the gateway verifier so signer and verifier never drift.

Pinned contract (honored)

GET /api/kernels/:kernelId → KernelDTO includes signingAddress: string | null.

Security invariant

• Proof bound to pcc-kernel-signing-key:${kernelId} → a signature captured for one kernel cannot register another kernel's address.
• Only cryptographically-proven addresses are stored; everything else is null.

Tests (all watched pass)

• gateway kernel-signing-address.test.ts — 7/7: valid → persisted + returned by /:id and list; mismatch / absent / garbage / cross-kernel replay → null; no-fields → null; set-once blocks overwrite. Proofs from a real viem test account.
• kernel kernel-registration-proof.test.ts — 4/4: round-trip recover, kernelId binding, per-keychain distinctness.
• No regression: existing register tests 10/10, @pcc/store store+device-registry 61/61. @pcc/store + @pcc/kernel build clean.

Follow-ups (out of scope)

1. Wire buildRegistrationProof into the live log-signing process's POST /api/kernels call (no existing TS daemon self-registers; pcc-node is Python/Ed25519, not the secp256k1 kernel key).
2. Owner-authenticated key rotation endpoint (this PR is set-once via the generic upsert).
3. Operator-ownership authz on who may bind a signer to a given kernelId.

🤖 Generated with Claude Code

[LamaSu]

Superseded by feat/optionc-machinelog (Option C tagged Ed25519-default keys, bulletin #306) — do not merge standalone.