EST RFC 7030 hardening master bundle Phases 2-4: end-to-end mTLS sibling

route + RFC 9266 channel binding + HTTP Basic enrollment-password +
per-source-IP failed-auth limit + per-(CN, sourceIP) sliding-window cap.

Two new shared packages so EST + Intune share infrastructure:
- internal/cms/ — RFC 9266 tls-exporter extractor (ExtractTLSExporter
  with stdlib-panic recovery for synthetic ConnectionStates) +
  CSR-side channel-binding parser via raw TBSCertificationRequestInfo
  walk (the stdlib's csr.Attributes can't represent the OCTET STRING
  binding value), VerifyChannelBinding composite, EmbedChannel-
  BindingAttribute fixture helper, typed sentinel errors for missing
  / mismatch / not-TLS-1.3 mapped to HTTP 400 / 409 / 426 in handler.
- internal/trustanchor/ — extracted from scep/intune/trust_anchor*.go
  so the EST mTLS sibling route + Intune dispatcher share the same
  SIGHUP-reloadable PEM bundle primitive. intune.TrustAnchorHolder
  is now `= trustanchor.Holder` (type alias) + NewTrustAnchorHolder =
  trustanchor.New (function alias) — every existing call site compiles
  unchanged. Intune's LoadTrustAnchor is a thin wrapper over
  trustanchor.LoadBundle. White-box tests moved to the new package.
- internal/ratelimit/ — extracted from scep/intune/rate_limit.go (this
  was Phase 4.1, in the same bundle). intune.PerDeviceRateLimiter
  is now a thin wrapper preserving the (subject, issuer)→key
  composition; EST handler reaches for SlidingWindowLimiter directly.

ESTHandler grew six optional fields wired by per-profile setters
(SetMTLSTrust / SetChannelBindingRequired / SetEnrollmentPassword /
SetSourceIPRateLimiter / SetPerPrincipalRateLimiter / SetLabelForLog)
plus four new mTLS-route methods (CACertsMTLS / SimpleEnrollMTLS /
SimpleReEnrollMTLS / CSRAttrsMTLS); shared internal pipeline
handleEnrollOrReEnroll(reEnroll, viaMTLS) keeps the auth/binding/
rate-limit gates DRY. New router method RegisterESTMTLSHandlers
registers /.well-known/est-mtls/<PathID>/{cacerts,simpleenroll,
simplereenroll,csrattrs}; AuthExemptDispatchPrefixes extends the
no-auth chain to /.well-known/est-mtls.

cmd/server/main.go's EST loop wires per-profile mTLS holder +
channel-binding policy + per-principal limiter + (when EnrollmentPassword
non-empty) Basic + source-IP limiter; new preflightESTMTLSClientCATrust-
Bundle returns *trustanchor.Holder so SIGHUP rotates the EST mTLS
bundle live without restart. SCEP + EST mTLS profiles now share a
single union mtlsUnionPoolForTLS passed to buildServerTLSConfigWithMTLS
(replaces the protocol-specific scepMTLSUnionPoolForTLS); per-handler
re-verify enforces "cert must chain to THIS profile's bundle" so
cross-protocol bleed is blocked at the application layer even though
the TLS layer trusts certs from either pool's union.

Phase 3.3 source-IP failed-Basic limiter defaults: 10 attempts / 1h
/ 50k tracked IPs (no env var; tunable in a follow-up). Phase 4.2
per-principal limiter cap from CERTCTL_EST_PROFILE_<NAME>_RATE_
LIMIT_PER_PRINCIPAL_24H (existing field, Phase 1 shipped).

New tests:
- internal/cms/channelbinding_test.go: extractor + CSR-side parser +
  composite + TLS-1.3 round-trip end-to-end + EmbedChannelBinding-
  Attribute round-trip
- internal/trustanchor/holder_test.go: parseBundlePEM white-box +
  LoadBundle + Holder Get/Pool/SetLabelForLog/Reload-happy/
  Reload-keeps-old-on-failure/Reload-keeps-old-on-expired/
  WatchSIGHUP-reloads-pool/WatchSIGHUP-stop-clean
- internal/api/handler/est_hardening_test.go: 16 named cases covering
  mTLS no-trust-pool 500 + no-cert 401 + cross-profile cert 401 +
  happy-path 200 + CACertsMTLS auth gate + CSRAttrsMTLS auth gate +
  channel-binding required-absent-rejected + not-required-absent-
  allowed + writeChannelBindingError mapping + Basic no-header 401
  + Basic wrong-password 401 + Basic correct-200 + Basic-no-password
  no-gate + per-IP failed-attempt lockout 429 + per-principal
  blocks-after-cap + different-principals-independent + no-limiter-
  unbounded.

Pre-commit verification (sandbox): gofmt clean, go vet clean
(excluding repository/postgres which the sandbox can't build —
disk-space testcontainers download), staticcheck clean for
cms/trustanchor/api/handler/api/router/scep/intune/ratelimit/
cmd/server, go test -short -count=1 green for cms/trustanchor/
api/handler/api/router/scep/intune/ratelimit/service. G-3
docs-drift guard reproduced locally clean (Phase 1 already
documented every new env var; Phases 2-4 added zero new env vars).

cmd/server/tls.go

@@ -136,21 +136,27 @@ func buildServerTLSConfig(holder *certHolder) *tls.Config {
 }
 
 // buildServerTLSConfigWithMTLS extends buildServerTLSConfig with a client-cert
-// trust pool for the SCEP RFC 8894 + Intune master bundle Phase 6.5 mTLS
-// sibling route. SCEP profiles that opt into mTLS each contribute their
-// trust bundle to the union pool here; the same TLS listener serves both
-// /scep[/<pathID>] (no client cert) and /scep-mtls/<pathID> (cert required
-// at the handler layer).
+// trust pool for the SCEP/EST mTLS sibling routes.
+//
+// SCEP RFC 8894 + Intune master bundle Phase 6.5 introduced this for the
+// /scep-mtls/<pathID> route; EST RFC 7030 hardening master bundle Phase 2
+// extended it so the same TLS listener also serves /.well-known/est-mtls/
+// <pathID>. Both protocols' mTLS profiles contribute their trust bundles
+// to a UNION pool that the caller (cmd/server/main.go) builds by walking
+// every enabled mTLS profile's bundle bytes once. The per-protocol
+// handlers re-verify against just THIS profile's bundle (so an EST-mTLS
+// bootstrap cert can't enroll against a SCEP-mTLS profile and vice versa).
 //
 // ClientAuth: VerifyClientCertIfGiven — request a cert during handshake; if
 // the client presents one, verify it against the union pool; if absent, the
 // request still reaches the handler and the per-route handler decides
 // whether to accept. Critical that we do NOT use RequireAndVerifyClientCert
-// here — that would break the standard /scep route (which is challenge-
-// password-only, no client cert expected).
+// here — that would break the standard /scep + /.well-known/est routes
+// (challenge-password-only / unauth-or-Basic, no client cert expected).
 //
-// Pass clientCAs == nil to disable mTLS (no profile opted in). The function
-// then returns the same shape as buildServerTLSConfig.
+// Pass clientCAs == nil to disable mTLS (no profile opted in across either
+// protocol). The function then returns the same shape as
+// buildServerTLSConfig.
 func buildServerTLSConfigWithMTLS(holder *certHolder, clientCAs *x509.CertPool) *tls.Config {
 	cfg := buildServerTLSConfig(holder)
 	if clientCAs != nil {