Phase 2 red: crypto primitive tests and stub modules

Tests for the entire crypto/ public surface, written against the API
shape declared in the README. The accompanying src/crypto/ modules are
stubs that throw 'not implemented' so the test files compile and tests
fail with clear errors rather than module-not-found.

Tests cover:
  * init() resolves and is idempotent
  * fromBase64 / toBase64 / toBase64URL round-trips, including URL-safe
    input with stripped padding (the form Ente uses for auth tokens)
  * deriveKEK matches sodium.crypto_pwhash with Argon2id parameters
  * deriveLoginSubkey matches sodium.crypto_kdf_derive_from_key with
    subkey id 1 and ctx 'loginctx', truncated to 16 bytes
  * decryptBox round-trips, rejects tampering, wrong key, wrong nonce
  * decryptSealed round-trips, rejects wrong keypair and tampering
  * Secretstream pull decrypts multi-chunk streams in order, exposes
    per-chunk tags, rejects tampering, wrong key, and out-of-order chunks
  * Constants STREAM_CHUNK_SIZE (4 MiB) and STREAM_CHUNK_OVERHEAD (17)

Tests are commented to serve as the canonical API documentation per the
README development workflow policy. Verified: 29 tests fail (red), 3
trivial constant tests pass; lint and fmt-check are green.

eslint.config.mjs is updated to honour the leading-underscore convention
for intentionally unused parameters (the stubs).

src/crypto/index.ts

@@ -0,0 +1,11 @@
+export { init } from "./sodium.js";
+export { fromBase64, toBase64, toBase64URL } from "./encoding.js";
+export { deriveKEK, deriveLoginSubkey } from "./kdf.js";
+export { decryptBox, decryptSealed } from "./box.js";
+export {
+    initStreamPull,
+    pullStreamChunk,
+    STREAM_CHUNK_OVERHEAD,
+    STREAM_CHUNK_SIZE,
+    type StreamPullState,
+} from "./stream.js";