feat(vault): decompose Vault into HSM capability interfaces

[canaan5]

The core interface-design step toward HSM support — making the vault façade ready for any HSM adapter to implement out of the box. Source-compatible (no method-set change), so it's safe to land before the v1 vault freeze.

What changed

vault.Vault is now the composition of three small capability interfaces:

type PINEncryptor interface { EncryptPINBlock(...) ([]byte, error) }   // issuer / trusted context (takes clear PIN)
type PINTranslator interface { TranslatePIN(...) ([]byte, error) }     // acquirer/switch (encrypted block in/out)
type Macer interface { GenerateMAC(...); VerifyMAC(...) }              // MAC under device-resident key

type Vault interface { PINEncryptor; PINTranslator; Macer }            // full façade (SoftVault, SealedVault)

So an adapter implements exactly what its hardware supports:

• a general-purpose PKCS#11 HSM → Macer (and possibly PINEncryptor)
• a payment HSM (Thales payShield / Futurex) → also PINTranslator

Callers type-assert for the narrowest capability they need:

tr, ok := v.(vault.PINTranslator)
if !ok { return errors.New("configured vault cannot translate PINs") }

The PIN-security contract, encoded in the type system

PINTranslator's doc makes the rule explicit: a conforming hardware implementation must re-encipher atomically inside the device so the clear PIN never leaves it (PCI PIN Security). An adapter that can't (e.g. stock PKCS#11, which has no atomic-translate mechanism) must not implement the interface — its absence is the signal that PIN translation isn't available. This resolves the finding from #11.

Verification

• Vault method set unchanged → SoftVault/SealedVault still satisfy it (compile-time assertions added for each capability).
• New TestCapabilityDetection shows a MAC-only adapter is correctly not usable as a PINTranslator.
• gofmt/vet clean; ./... builds; vault/flow/gateway pass under -race.

Next (separate PRs)

• Update adapters/pkcs11 (feat(adapters/pkcs11): HSM-backed MAC (vault.Macer) verified against SoftHSM2 #11) to implement Macer against SoftHSM2 in CI; keep TranslatePIN unimplemented (correct per the contract).
• Scaffold adapters/payshield (PINTranslator + Macer) against a payShield simulator.

Touches CHANGELOG.md [Unreleased] like #8/#9 — trivial conflict for whichever merges later.