Batch A: blob storage layer, MCIAS auth, OCI token endpoint

Phase 2 — internal/storage/:
Content-addressed blob storage with atomic writes via rename.
BlobWriter stages data in uploads dir with running SHA-256 hash,
commits by verifying digest then renaming to layers/sha256/<prefix>/<hex>.
Reader provides Open, Stat, Delete, Exists with digest validation.

Phase 3 — internal/auth/ + internal/server/:
MCIAS client with Login and ValidateToken, 30s SHA-256-keyed cache
with lazy eviction and injectable clock for testing. TLS 1.3 minimum
with optional custom CA cert.
Chi router with RequireAuth middleware (Bearer token extraction,
WWW-Authenticate header, OCI error format), token endpoint (Basic
auth → bearer exchange via MCIAS), and /v2/ version check handler.

52 tests passing (14 storage + 9 auth + 9 server + 20 existing).

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

internal/auth/claims.go

@@ -0,0 +1,27 @@
+package auth
+
+import "context"
+
+// Claims represents the validated identity and roles extracted from an
+// MCIAS token.
+type Claims struct {
+	Subject     string
+	AccountType string
+	Roles       []string
+}
+
+// claimsKey is an unexported type used as the context key for Claims,
+// preventing collisions with keys from other packages.
+type claimsKey struct{}
+
+// ContextWithClaims returns a new context carrying the given Claims.
+func ContextWithClaims(ctx context.Context, c *Claims) context.Context {
+	return context.WithValue(ctx, claimsKey{}, c)
+}
+
+// ClaimsFromContext extracts Claims from the context. It returns nil if
+// no claims are present.
+func ClaimsFromContext(ctx context.Context) *Claims {
+	c, _ := ctx.Value(claimsKey{}).(*Claims)
+	return c
+}