feat(dig-runtime): wallet-only start + native read-crypto FFI

[MichaelTaylor3d]

TLDR

Adds two additive C-ABI surfaces to the dig-runtime cdylib so the DIG Browser can (a) start the runtime wallet-only — no in-process node engine — and (b) verify+decrypt .dig content by calling the digstore Rust read-crypto DIRECTLY via FFI, NOT wasm (wasm is webpages-only — @MT-Dev's ruling on super #44). Implements super-repo #47 and the Rust side of #44 item 2. All existing exports unchanged.

What changed (crates/dig-runtime/src/lib.rs)

• dig_runtime_start_wallet() — wallet-only start (#47). Brings up the wallet host + tokio runtime with NO node engine (no P2P / cache / dig_rpc dispatch). The existing full dig_runtime_start() is unchanged; the FIRST dig_runtime_start* call fixes the mode (OnceLock). In wallet-only mode dig_rpc returns a well-formed JSON-RPC error (code -32000, "node engine not available: dig-runtime started wallet-only") rather than spinning up a node.
• dig_read_verify_decrypt() / dig_bytes_free() — native read-crypto FFI (#44 item 2). Exports the digstore-core .dig verify+decrypt as C-ABI — the SAME Rust the webpage dig-client-wasm wraps (ONE impl, two bindings). Verifies the served ciphertext's Merkle inclusion against the chain-anchored root, THEN AES-256-GCM-SIV decrypts, fail-closed (DIG_READ_OK/BAD_INPUT/VERIFY_FAILED/DECRYPT_FAILED/INTERNAL). Needs no runtime and no node engine — pure crypto over bytes the caller already fetched from an external node (§5.3). Byte-parity with the wasm's decryptResource (same canonical_resource_urn + derive_decryption_key + inclusion-proof core).

digstore-core is pinned to the same rev (bcb84a6…) dig-node-core/dig-wallet use, so cargo unifies to one crypto impl.

FFI contract (for the dig-browser C++ loader)

// wallet-only start (browser calls this at startup instead of dig_runtime_start)
void dig_runtime_start_wallet(void);

// read-crypto: verify Merkle inclusion vs the chain-anchored root, then decrypt. fail-closed.
int32_t dig_read_verify_decrypt(
    const char* store_id_hex,      // 64-hex (required)
    const char* resource_key,      // resource path; "" => index.html (required)
    const uint8_t* ciphertext, size_t ciphertext_len,  // plain concat of chunk ciphertexts
    const char* proof_b64,         // base64 X-Dig-Inclusion-Proof (required)
    const char* trusted_root_hex,  // 64-hex chain-anchored root (required)
    const char* salt_hex,          // 64-hex private-store salt, or NULL/"" for public
    const uint32_t* chunk_lens, size_t chunk_lens_len, // per-chunk ct byte lens; NULL/0 => single
    uint8_t** out_ptr, size_t* out_len);               // heap plaintext; free w/ dig_bytes_free
void dig_bytes_free(uint8_t* ptr, size_t len);
// status: 0 OK, 1 BAD_INPUT, 2 VERIFY_FAILED, 3 DECRYPT_FAILED, 4 INTERNAL

Tests (TDD) — 14 pass

• #47 split: build_runtime(false) constructs no node engine (tokio still live); build_runtime(true) does; dig_rpc with no engine returns a JSON-RPC error, never a panic.
• #44 read-crypto: public single-chunk round-trip; empty key ⇒ index.html; private-salt round-trip (+ wrong salt ⇒ DECRYPT_FAILED); multi-chunk; tampered ciphertext ⇒ VERIFY_FAILED; decoy/wrong-root ⇒ VERIFY_FAILED; bad hex ⇒ BAD_INPUT; null out-params ⇒ BAD_INPUT.
• Existing full-runtime FFI round-trips (dig_rpc, dig_wallet_rpc envelope, null-pointer safety) unchanged.

Verification (this session)

• cargo test -p dig-runtime → 14 passed, 0 failed.
• cargo clippy -p dig-runtime --all-targets -- -D warnings → clean (fixed a cast_slice_from_raw_parts lint in dig_bytes_free).
• cargo fmt --all -- --check → clean.

Blast radius

dig-runtime is a leaf cdylib — nothing in the workspace depends on it (it is the browser's DLL). All changes are additive: new exports + an internal with_node split of the runtime constructor; existing dig_rpc/dig_wallet_rpc/dig_free semantics preserved. Consumer is the dig-browser C++ loader (super #44 Part B), migrated separately.

Docs

• SPEC.md §15 reworked (two start modes + the three FFI surfaces), new §15.1 (read-crypto FFI: signatures, inputs, status codes, buffer ownership), §1.3 corrected (browser starts wallet-only), conformance-index row 15 updated.
• SYSTEM.md (superproject) note: line ~123 ("exactly ONE read-crypto implementation (the shared wasm)") should be updated by the orchestrator to "one Rust impl, two bindings (native FFI for the browser, wasm for webpages)" — out of this repo's scope.

Version

dig-runtime 0.1.0 → 0.2.0 (minor — additive feat, no breaking change to existing exports).

Refs super-repo #44, #47.