feat(middleware): per-request context object (requestState)

Adds requeststate.go and threads a *requestState through the
ServeHTTP -> processAuthorizedRequestRS -> forwardAuthorized path.
rs is allocated once at the top of ServeHTTP, populates SessionData
field snapshots under a SINGLE sd.sessionMutex.RLock, and caches the
MetadataSnapshot. Downstream handlers read the cached fields instead
of calling session.GetX() / t.metadataSnap() repeatedly.

Why
---
Under Yaegi each method dispatch (including RWMutex.RLock) costs
~1-5ms of interpreter overhead. SessionData getters each take an
RLock on sd.sessionMutex; the previous hot path called 5-7 of them
per request (GetAuthenticated, GetAccessToken, GetIDToken,
GetRefreshToken, GetUserIdentifier, plus the same set again inside
processAuthorizedRequest). With one batched RLock + cached fields,
that drops to a single RLock for the whole handler chain.

This is scoped — not a wholesale architectural refactor:

* requestState is per-request (alloc at ServeHTTP entry, dropped on
  return). It is NOT a shared cache and never escapes the request.
* The original processAuthorizedRequest is kept unchanged for any
  callers we don't migrate this round (bearer path, callback
  handlers, expired-token handlers). New code path is the RS-aware
  processAuthorizedRequestRS, which middleware.ServeHTTP now uses for
  the happy authenticated-and-not-needing-refresh case.
* Cross-request caches (tokenCache, JWKCache, sessionEntries,
  sessionInvalidationCache) are unchanged. rs is additive, not a
  replacement.

What this does NOT change
-------------------------
* The refresh path still calls session.GetX() in middleware.go
  (handleExpiredToken, refreshToken, defaultInitiateAuthentication)
  because those flows can mutate session state and a stale rs would
  be wrong.
* validateStandardTokens still has its own session.GetX() calls.
  Deep plumbing into the token-verification path is a follow-up.
* No semantic changes to authentication, refresh, or session
  lifecycle — only the read path is optimised.

All tests pass with -race; golangci-lint clean.

requeststate.go

@@ -0,0 +1,71 @@
+// Package traefikoidc provides OIDC authentication middleware for Traefik.
+// requestState bundles read-mostly fields for a single ServeHTTP call.
+package traefikoidc
+
+import "net/http"
+
+// requestState is a per-request context object allocated at the top of
+// ServeHTTP and threaded through to downstream handlers. It caches values
+// that would otherwise require a Yaegi-dispatched lock acquisition each time
+// they're read:
+//
+//   - The metadata snapshot (atomic.Value.Load once, not per-handler).
+//   - SessionData getter results (one RLock on sd.sessionMutex covers all
+//     fields, instead of 5-7 separate RLock/RUnlock pairs scattered through
+//     the handler chain).
+//
+// The struct is alloc'd at request entry, populated under at most one RLock
+// of sd.sessionMutex, and discarded at request exit. It is NOT shared across
+// requests and never written from another goroutine, so no synchronization
+// on its fields is required.
+//
+// Cross-request global caches (tokenCache, JWKCache, sessionEntries,
+// sessionInvalidationCache) remain — they're orthogonal. requestState's job
+// is to eliminate redundant per-handler reads of values that don't change
+// within a single request.
+type requestState struct {
+	// Globals snapshotted once.
+	metadata *MetadataSnapshot
+
+	// SessionData fields snapshotted under one RLock. The pointer to the
+	// SessionData is retained so handlers that genuinely need to mutate
+	// (Save, Clear, etc.) still have access.
+	session *SessionData
+
+	authenticated     bool
+	accessToken       string
+	idToken           string
+	refreshToken      string
+	userIdentifier    string
+	createdAtUnixSec  int64
+
+	// Output: scheme/host/redirect path determined at top of ServeHTTP.
+	scheme      string
+	host        string
+	redirectURL string
+
+	// Carry the next handler so forwardAuthorized doesn't need to close over t.
+	next http.Handler
+}
+
+// captureSession populates requestState's SessionData-derived fields under a
+// single RLock of sd.sessionMutex. Returns the populated rs for chaining.
+//
+// Replaces a sequence of SessionData.GetX() calls each of which acquires
+// sd.sessionMutex.RLock(). Under Yaegi each RLock costs ~1-5ms of
+// interpreter dispatch; batching saves the rest.
+func (rs *requestState) captureSession(sd *SessionData) *requestState {
+	if sd == nil {
+		return rs
+	}
+	rs.session = sd
+	sd.sessionMutex.RLock()
+	rs.authenticated = sd.getAuthenticatedUnsafe()
+	rs.accessToken = sd.getAccessTokenUnsafe()
+	rs.idToken = sd.getIDTokenUnsafe()
+	rs.refreshToken = sd.getRefreshTokenUnsafe()
+	rs.userIdentifier = sd.getUserIdentifierUnsafe()
+	rs.createdAtUnixSec = sd.getCreatedAtUnsafe()
+	sd.sessionMutex.RUnlock()
+	return rs
+}