feat(auth): initial implementation — authmw and rbac (v1.0.0)

Introduces code.nochebuena.dev/einherjar/auth — the provider-agnostic HTTP
authentication and authorization layer of the Einherjar framework. Absorbs
two micro-lib packages (httpauth, rbac) as sub-packages, replacing the
Identity-only context model with a SecurityBag-native design and adding a
composable enrichment chain.

authmw:
- BagEnricher function type — enriches the request-scoped SecurityBag after
  the base Identity is built; registered via WithBagEnricher; multiple
  enrichers run in order, each receiving the bag from the previous
- IdentityEnricher interface — application-layer contract for loading user
  data from uid+claims
- EnrichmentMiddleware — builds SecurityBag from uid+claims, runs enricher
  chain, stores via security.SetBagInContext; 401 on missing uid, 500 on
  enricher error; routes all errors through httputil.Error
- AuthzMiddleware — per-route permission gate; 401 on missing identity,
  403 on provider error (fail-closed) or insufficient permissions
- EnrichOpt type + WithTenantHeader (reads TenantID from header, implemented
  as a BagEnricher) + WithBagEnricher (registers custom enrichers for
  hardware IDs, grant codes, or any bag attribute)
- SetTokenData / GetClaims — integration contract for auth-jwt / auth-firebase

rbac:
- NewClaimsPermissionProvider — reads flat JWT claim bitmasks from context;
  wildcard "*" fallback; handles int64/float64/json.Number; zero DB calls
- NewCachedPermissionProvider — TTL cache wrapping any PermissionProvider;
  default key "rbac:{uid}:{resource}" or "rbac:{tenantID}:{uid}:{resource}";
  TenantID sourced from SecurityBag automatically; accepts ...CachedOpt
- CachedOpt type + WithCacheKey — overrides the key function for extra
  dimensions (hardware IDs, grant codes read from bag attributes)
- NewChainPermissionProvider — tries providers in order; first non-zero wins;
  errors short-circuit; typical pattern: claims → cached DB fallback
- Cache interface — pluggable backend satisfied by cache-valkey via duck typing

Compliance test (package auth_test) enforces CT-6 (≤1 exported TypeSpec/file),
compile-time interface satisfaction, and behavioural coverage across the full
middleware and provider surface: enrichment success/failure, tenant header,
custom BagEnricher, bag-in-context, authz allowed/denied/error, claims
hit/wildcard/missing/float64, cached hit/miss/error/tenant-key/custom-key,
chain first-non-zero/fallthrough/error.

Depends on contracts v1.0.0, core v1.0.0, web v1.0.0.

- identifiable.go: package-level Module variable (observability.Identifiable) for version
  identification — auth is middleware-only; not registered with the launcher

rbac/doc.go

@@ -0,0 +1,43 @@
+// Package rbac provides permission provider implementations for the Einherjar
+// authorization system. All constructors return [security.PermissionProvider]
+// from contracts/security — no new types are defined here.
+//
+// # Permission providers
+//
+// Three strategies compose into a complete authorization pipeline:
+//
+//	// Fast-path: reads pre-computed bitmasks from JWT claims in context.
+//	// Pass authmw.GetClaims so rbac does not import authmw directly.
+//	claims := rbac.NewClaimsPermissionProvider("perms", authmw.GetClaims)
+//
+//	// DB + cache: wraps any provider with TTL caching. Cache key is
+//	// automatically scoped by TenantID for multi-tenant deployments.
+//	cached := rbac.NewCachedPermissionProvider(dbProvider, valkeyCache, 5*time.Minute)
+//
+//	// Chain: tries claims first, falls through to DB on miss.
+//	chain := rbac.NewChainPermissionProvider(claims, cached)
+//
+// # Cache key customization
+//
+// When additional bag attributes must be part of the cache key (e.g. hardware IDs):
+//
+//	const KeyHardwareID = "hardware_id"
+//
+//	cached := rbac.NewCachedPermissionProvider(dbProvider, cache, 5*time.Minute,
+//	    rbac.WithCacheKey(func(bag security.SecurityBag, uid, resource string) string {
+//	        hwID, _ := bag.Get(KeyHardwareID)
+//	        return fmt.Sprintf("rbac:%s:%s:%v:%s", bag.Identity().TenantID, uid, hwID, resource)
+//	    }),
+//	)
+//
+// # Cache interface
+//
+// [Cache] is satisfied by einherjar/cache-valkey via Go duck typing.
+// No import of auth/rbac is needed by the cache implementation.
+//
+// # Multi-tenant
+//
+// [NewCachedPermissionProvider] automatically includes TenantID in the cache key
+// when [security.Identity.TenantID] is non-empty in the request bag.
+// Populate TenantID via [authmw.WithTenantHeader] in EnrichmentMiddleware.
+package rbac