feat: add crypto_hash family, memzero, byte additional_data for AEAD, and crypto_secretstream_xchacha20poly1305

[slowsats]

Four additions to the native surface, one commit each, all matching libsodium-wrappers-sumo@0.8.4 semantics (argument shapes, return types, and failure behavior verified against its source):

crypto_hash, crypto_hash_sha256, crypto_hash_sha512 (+ _BYTES constants; web side re-exports the sumo-only spellings).
memzero — sodium_memzero over the ArrayBuffer's actual backing memory, honoring byteOffset/byteLength (a subarray test proves sibling bytes survive). Relates to #76.
additional_data: string | Uint8Array | null on crypto_aead_xchacha20poly1305_ietf_encrypt/_decrypt — the string path is byte-for-byte unchanged (existing vector tests pass unmodified); byte AAD authenticates byte-exact; null AAD (previously rejected natively) now matches the reference, pinned against a ciphertext generated from libsodium-wrappers-sumo@0.8.4 itself.
The crypto_secretstream_xchacha20poly1305 family (keygen, init_push → {state, header}, push, init_pull, pull → MessageTag | false) with _ABYTES/_HEADERBYTES/KEYBYTES and the four TAG* constants. pull reproduces the reference exactly: false on any cryptographic failure without advancing the state, TypeError('cipher is too short') only under ABYTES. Design note for review: the repo had no prior pattern for stateful bindings, so the stream state crosses JSI as an opaque ArrayBuffer holding the raw state struct, advanced in place — no handle registry, no lifetime management, no leak (the reference leaks states in wasm memory by design); trade-off comments are on the C++ helpers and the TS family. Happy to add rekey and _MESSAGEBYTES_MAX for family completeness if wanted.

Tests: example-app cases for everything (FIPS 180-4 digests, multi-chunk secretstream round-trip with all four tags, truncation/reorder/tamper failures, byte-vs-string AAD equivalence). The example suite passes on Android — including on physical arm64 hardware (Pixel 7 Pro) — and the web e2e suite (Playwright) independently validates every new test's expectations against the real libsodium-wrappers implementation. iOS is implemented in the shared C++ (compiled by the existing podspec glob; the vendored xcframework exports every required symbol) — CI is the verification channel, as this was developed on Linux.