v0.15.1

Adds the cryptographic bindings needed to implement IRCv3 WEBPUSH (https://ircv3.net/specs/extensions/webpush) on Enclave or any other Sigil consumer wanting Web Push notification delivery.

Native bindings (mbedTLS-backed; primitives already compiled in): - ecdsa-p256-generate-keypair / sign / verify (VAPID JWT, RFC 8292) - ecdh-p256-shared-secret (RFC 8291) - aes-128-gcm-encrypt / decrypt (RFC 8291 payload envelope)

Pure-Sigil additions: - base64url-encode / base64url-decode (RFC 4648 § 5) - hkdf-sha256-extract / -expand / one-shot (RFC 5869)

KAT tests cover RFC 5903 ECDH, NIST SP 800-38D AES-GCM, RFC 5869 HKDF, RFC 8291 § 5 end-to-end WebPush envelope, plus tamper-detection (signature, AAD, ciphertext, plaintext, off-curve pubkey).

Note: standalone 'sigil test' in this worktree fails to load v0.15.0+ native bindings due to a test-runner cache issue documented in tasks/sigil-crypto-test-runner-stale-native-cache; consumer-side builds are unaffected and exercise the bindings correctly. Fixing the test runner is the immediate follow-up.