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

CHANGELOG.md

@@ -0,0 +1,47 @@
+# Changelog
+
+## v1.0.0
+
+Initial release.
+
+### `authmw`
+
+- `BagEnricher` type — `func(bag security.SecurityBag, r *http.Request) security.SecurityBag`;
+  enriches the request-scoped SecurityBag after the base Identity is built. Register via
+  `WithBagEnricher`. Multiple enrichers run in registration order, each receiving the bag
+  returned by the previous one.
+- `SetTokenData` — integration contract for provider packages (auth-jwt, auth-firebase).
+  Stores uid and raw claims in context via typed keys; consumed by `EnrichmentMiddleware`.
+- `GetClaims` — exported accessor for raw token claims stored by `SetTokenData`. Available
+  to custom `IdentityEnricher` implementations and `ClaimsPermissionProvider`.
+- `EnrichmentMiddleware` — builds a `security.SecurityBag` from uid+claims. Calls the
+  application `IdentityEnricher`, wraps the Identity in a SecurityBag, runs all registered
+  `BagEnricher` functions in order, then stores the bag via `security.SetBagInContext`.
+  Accepts `logging.Logger`; routes errors through `httputil.Error` (401 on missing token,
+  500 on enricher failure).
+- `AuthzMiddleware` — per-route permission gate. Returns 401 on missing identity, 403 on
+  provider error or insufficient permissions (fail-closed).
+- `IdentityEnricher` interface — implemented by the application to load user data from uid+claims.
+- `EnrichOpt` type — `func(*enrichConfig)`.
+- `WithTenantHeader(header string) EnrichOpt` — reads Identity.TenantID from a named request
+  header. Implemented as a `BagEnricher` internally.
+- `WithBagEnricher(fn BagEnricher) EnrichOpt` — registers a custom enricher. Use for any
+  attribute beyond TenantID: hardware IDs, grant codes, etc.
+
+### `rbac`
+
+- `NewClaimsPermissionProvider` — reads pre-computed bitmasks from JWT claims in context.
+  Flat format: `claims[claimsKey][resource] = mask`. Wildcard `"*"` fallback.
+  Handles int64, float64, json.Number.
+- `NewCachedPermissionProvider` — wraps any `security.PermissionProvider` with TTL caching.
+  Default cache key: `"rbac:{uid}:{resource}"` (single-tenant) or
+  `"rbac:{tenantID}:{uid}:{resource}"` (multi-tenant). TenantID sourced from the SecurityBag
+  in context automatically. Accepts `...CachedOpt` for customization.
+- `CachedOpt` type — `func(*cachedConfig)`.
+- `WithCacheKey(fn func(security.SecurityBag, string, string) string) CachedOpt` — overrides
+  the default cache key function. Use when additional bag attributes (hardware IDs, grant codes)
+  must be part of the key.
+- `NewChainPermissionProvider` — tries providers in order; returns first non-zero mask. Errors
+  short-circuit.
+- `Cache` interface — pluggable cache backend. Satisfied by `einherjar/cache-valkey` via duck
+  typing.