crypto/signer: introduce Signer interface; refactor local issuer to use it

This is a load-bearing internal refactor with no user-visible behavior
change. The new internal/crypto/signer package abstracts CA private-key
signing behind a Signer interface (embeds stdlib crypto.Signer + adds
Algorithm()). The local issuer now consumes this interface; the
historical c.caKey crypto.Signer field is renamed c.caSigner signer.Signer.

What landed:

  * internal/crypto/signer/ — new stdlib-only package
    - Signer interface: crypto.Signer + Algorithm()
    - Algorithm enum: RSA-2048, RSA-3072, RSA-4096, ECDSA-P256, ECDSA-P384
    - Driver interface: Load / Generate / Name
    - FileDriver: production driver, wraps file-on-disk PEM, hooks for
      DirHardener + Marshaler so the local package can inject Bundle 9
      keystore.ensureKeyDirSecure + keymem.marshalPrivateKeyAndZeroize
    - MemoryDriver: in-memory test driver; safe for concurrent use
    - parse.go: ParsePrivateKey moved here from local.go (PKCS#1, SEC 1, PKCS#8)
    - 91.6% coverage (gate ≥85)

  * internal/connector/issuer/local/local.go — refactor
    - Rename c.caKey crypto.Signer → c.caSigner signer.Signer
    - Rewire 4 signing call sites: leaf cert (line ~613), CRL (~849),
      OCSP response (~887), CA bootstrap (~482) — all access the
      interface; the bootstrap also switches to interface-level
      Public() + Signer
    - Wrap freshly-generated and freshly-loaded keys; reject Ed25519
      and other unsupported algorithms at load time (was silently
      accepted before, would have failed at first sign)
    - Delete the duplicated parsePrivateKey helper (single source of
      truth now lives in the signer package)
    - Update the L-014 threat-model comment block (lines 1-29) with a
      forward-reference paragraph: file-on-disk caveats apply only to
      FileDriver-backed signers; alternative drivers close that leg
    - Coverage 86.7 → 86.5 (above CI floor of 86); the 0.2pp drop is
      mechanical from deleting parsePrivateKey, partially recovered by
      a new test pinning the Wrap error path

  * internal/crypto/signer/equivalence_test.go — Phase 3 safety net
    - RSA byte-strict equality for leaf certs / CRLs / OCSP responses
      (PKCS#1 v1.5 is deterministic)
    - ECDSA TBS-strict equality (signature differs because of random k)
    - Both signatures independently validate against the CA
    - Negative sentinel proves the equivalence checker isn't trivially-
      passing

  * docs/architecture.md — new 'CA Signing Abstraction' section under
    Security Model, with ASCII diagram of FileDriver / MemoryDriver /
    future PKCS11Driver / future CloudKMSDriver

  * Test file mechanical edits (only):
    - bundle9_coverage_test.go: parsePrivateKey → signer.ParsePrivateKey
      (function moved, not behavior changed)
    - local_test.go: append one targeted test
      (TestSubCA_LoadCAFromDisk_RejectsUnsupportedKeyAlgorithm) that
      pins the new Wrap error path I introduced — recovers coverage
      cost of the deletion above

What did NOT change (verified empty diffs):
  * api/openapi.yaml
  * migrations/
  * internal/connector/issuer/interface.go
  * go.mod / go.sum (no new dependencies; stdlib only)

This refactor is the prerequisite for three downstream items:
  - PKCS#11/HSM driver (V3-Pro)
  - CRL/OCSP responder (V2)
  - SSH CA lifecycle (V2)

Each of those adds a new signing call site. Doing the abstraction now
costs once; deferring would cost three times.

internal/connector/issuer/local/bundle9_coverage_test.go

@@ -23,6 +23,7 @@ import (
 	"time"
 
 	"github.com/shankar0123/certctl/internal/connector/issuer"
+	"github.com/shankar0123/certctl/internal/crypto/signer"
 )
 
 // Bundle-9 / Audit H-010 + L-002 + L-003 + L-012 + M-028 regression suite.
@@ -133,7 +134,7 @@ func TestGetRenewalInfo_ReturnsNilNil(t *testing.T) {
 func TestParsePrivateKey_RSAPKCS1(t *testing.T) {
 	k := mustGenRSAKey(t)
 	der := x509.MarshalPKCS1PrivateKey(k)
-	signer, err := parsePrivateKey(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: der})
+	signer, err := signer.ParsePrivateKey(&pem.Block{Type: "RSA PRIVATE KEY", Bytes: der})
 	if err != nil {
 		t.Fatalf("parsePrivateKey RSA PKCS1: %v", err)
 	}
@@ -148,7 +149,7 @@ func TestParsePrivateKey_ECPrivateKey(t *testing.T) {
 	if err != nil {
 		t.Fatalf("marshal: %v", err)
 	}
-	signer, err := parsePrivateKey(&pem.Block{Type: "EC PRIVATE KEY", Bytes: der})
+	signer, err := signer.ParsePrivateKey(&pem.Block{Type: "EC PRIVATE KEY", Bytes: der})
 	if err != nil {
 		t.Fatalf("parsePrivateKey EC: %v", err)
 	}
@@ -163,7 +164,7 @@ func TestParsePrivateKey_PKCS8RSA(t *testing.T) {
 	if err != nil {
 		t.Fatalf("marshal pkcs8: %v", err)
 	}
-	signer, err := parsePrivateKey(&pem.Block{Type: "PRIVATE KEY", Bytes: der})
+	signer, err := signer.ParsePrivateKey(&pem.Block{Type: "PRIVATE KEY", Bytes: der})
 	if err != nil {
 		t.Fatalf("parsePrivateKey PKCS8: %v", err)
 	}
@@ -178,7 +179,7 @@ func TestParsePrivateKey_PKCS8ECDSA(t *testing.T) {
 	if err != nil {
 		t.Fatalf("marshal pkcs8: %v", err)
 	}
-	signer, err := parsePrivateKey(&pem.Block{Type: "PRIVATE KEY", Bytes: der})
+	signer, err := signer.ParsePrivateKey(&pem.Block{Type: "PRIVATE KEY", Bytes: der})
 	if err != nil {
 		t.Fatalf("parsePrivateKey PKCS8 ECDSA: %v", err)
 	}
@@ -188,7 +189,7 @@ func TestParsePrivateKey_PKCS8ECDSA(t *testing.T) {
 }
 
 func TestParsePrivateKey_UnknownType(t *testing.T) {
-	_, err := parsePrivateKey(&pem.Block{Type: "DSA PRIVATE KEY", Bytes: []byte{1, 2, 3}})
+	_, err := signer.ParsePrivateKey(&pem.Block{Type: "DSA PRIVATE KEY", Bytes: []byte{1, 2, 3}})
 	if err == nil {
 		t.Fatal("expected error on unknown PEM type")
 	}
@@ -198,7 +199,7 @@ func TestParsePrivateKey_UnknownType(t *testing.T) {
 }
 
 func TestParsePrivateKey_MalformedPKCS8(t *testing.T) {
-	_, err := parsePrivateKey(&pem.Block{Type: "PRIVATE KEY", Bytes: []byte{0xff, 0xff, 0xff}})
+	_, err := signer.ParsePrivateKey(&pem.Block{Type: "PRIVATE KEY", Bytes: []byte{0xff, 0xff, 0xff}})
 	if err == nil {
 		t.Fatal("expected error on malformed PKCS8")
 	}
@@ -855,4 +856,3 @@ func TestGenerateCertificate_WithMaxTTLCap(t *testing.T) {
 		t.Errorf("MaxTTL cap not honored, got window %s", got)
 	}
 }
-