ADR-003 wiring: Bearer token + IP allowlist middleware

Implements the auth posture from ADR-003 (commit 0d18ffa). Two
independent layers — Bearer token (constant-time compare via
crypto/subtle) and IP allowlist (CIDR set) — composed in shared.Run
so every binary inherits the same gate without per-binary wiring.

Together with the bind-gate from commit 6af0520, this mechanically
closes audit risks R-001 + R-007:
  - non-loopback bind without auth.token = startup refuse
  - non-loopback bind WITH auth.token + override env = allowed
  - loopback bind = all gates open (G0 dev unchanged)

internal/shared/auth.go (NEW)
  RequireAuth(cfg AuthConfig) returns chi-compatible middleware.
  Empty Token + empty AllowedIPs → pass-through (G0 dev mode).
  Token-only → 401 Bearer mismatch.
  AllowedIPs-only → 403 source IP not in CIDR set.
  Both → both gates apply.
  /health bypasses both layers (load-balancer / liveness probes
  shouldn't carry tokens).

  CIDR parsing pre-runs at boot; bare IP (no /N) treated as /32 (or
  /128 for IPv6). Invalid entries log warn and drop, fail-loud-but-
  not-fatal so a typo doesn't kill the binary.

  Token comparison: subtle.ConstantTimeCompare on the full
  "Bearer <token>" wire-format string. Length-mismatch returns 0
  (per stdlib spec), so wrong-length tokens reject without timing
  leak. Pre-encoded comparison slice stored in the middleware
  closure — one allocation per request.

  Source-IP extraction prefers net.SplitHostPort fallback to
  RemoteAddr-as-is for httptest compatibility. X-Forwarded-For
  support is a follow-up when a trusted proxy fronts the gateway
  (config knob TBD per ADR-003 §"Future").

internal/shared/server.go
  Run signature: gained AuthConfig parameter (4th arg).
  /health stays mounted on the outer router (public).
  Registered routes go inside chi.Group with RequireAuth applied —
  empty config = transparent group.
  Added requireAuthOnNonLoopback startup check: non-loopback bind
  with empty Token = refuse to start (cites R-001 + R-007 by name).

internal/shared/config.go
  AuthConfig type added with TOML tags. Fields: Token, AllowedIPs.
  Composed into Config under [auth].

cmd/<svc>/main.go × 7 (catalogd, embedd, gateway, ingestd, queryd,
storaged, vectord, mcpd is unaffected — stdio doesn't bind a port)
  Each call site adds cfg.Auth as the 4th arg to shared.Run. No
  other changes — middleware applies via shared.Run uniformly.

internal/shared/auth_test.go (12 test funcs)
  Empty config pass-through, missing-token 401, wrong-token 401,
  correct-token 200, raw-token-without-Bearer-prefix 401, /health
  always public, IP allowlist allow + reject, bare IP /32, both
  layers when both configured, invalid CIDR drop-with-warn, RemoteAddr
  shape extraction. The constant-time comparison is verified by
  inspection (comments in auth.go) plus the existence of the
  passthrough test (length-mismatch case).

Verified:
  go test -count=1 ./internal/shared/  — all green (was 21, now 33 funcs)
  just verify                            — vet + test + 9 smokes 33s
  just proof contract                    — 53/0/1 unchanged

Smokes + proof harness keep working without any token configuration:
default Auth is empty struct → middleware is no-op → existing tests
pass unchanged. To exercise the gate, operators set [auth].token in
lakehouse.toml (or, per the "future" note in the ADR, via env var).

Closes audit findings:
  R-001 HIGH — fully mechanically closed (was: partial via bind gate)
  R-007 MED  — fully mechanically closed (was: design-only ADR-003)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

internal/shared/config.go

@@ -28,6 +28,7 @@ type Config struct {
 	Embedd   EmbeddConfig  `toml:"embedd"`
 	S3       S3Config      `toml:"s3"`
 	Log      LogConfig     `toml:"log"`
+	Auth     AuthConfig    `toml:"auth"`
 }
 
 // IngestConfig adds ingestd-specific knobs. ingestd needs to PUT
@@ -126,6 +127,27 @@ type LogConfig struct {
 	Level string `toml:"level"`
 }
 
+// AuthConfig is the inter-service auth posture from ADR-003.
+// Token is a Bearer token; empty means "no auth" (G0 dev mode).
+// AllowedIPs is a list of CIDRs (or bare IPs treated as /32);
+// empty means "any source IP."
+//
+// Both layers operate independently when set:
+//   - Token + AllowedIPs both empty  → middleware is a no-op
+//   - Token only                     → 401 unless Bearer matches
+//   - AllowedIPs only                → 403 unless r.RemoteAddr in CIDR
+//   - Both                           → both gates apply
+//
+// The startup gate in shared.Run refuses to start with non-loopback
+// bind AND empty Token — that's the audit's R-001 + R-007 worst
+// case (no auth, world-reachable). LH_<SVC>_ALLOW_NONLOOPBACK=1 still
+// bypasses the bind gate for explicit dev cases; the auth gate is
+// independent of that bypass and is the real production guard.
+type AuthConfig struct {
+	Token      string   `toml:"token"`
+	AllowedIPs []string `toml:"allowed_ips"`
+}
+
 // DefaultConfig returns the G0 dev defaults. Ports are shifted to
 // 3110+ to coexist with the live Rust lakehouse on 3100/3201-3204
 // during the migration. G5 cutover flips gateway back to 3100.