test(crypto): add keys + primitives coverage with Argon2id parameter lock

[h4yfans]

Closes Phase 1 §1.4 of .claude/plans/tech-debt-remediation.md.

Summary

Dedicated coverage for keys.ts (205L) + primitives.ts (16L) with real libsodium, including an Argon2id regression lock via inline snapshot.

Key features

• Argon2id parameter regression lock via toMatchInlineSnapshot() on a fixed (seed=64×0x42, salt=0x00..0xff) pair + a separate crypto_pwhash arg-spy asserting ARGON2_PARAMS.OPS_LIMIT/MEMORY_LIMIT/ALG_ARGON2ID13. Inline comment documents the parallelism=1 deviation from RFC 9106 spec=4 (per MEMORY.md).
• Real libsodium end-to-end (no crypto mocks); keytar mocked only for keychain isolation.

Coverage

keys.test.ts (505L) — Argon2id lock, deriveKey context separation, deriveMasterKey snapshot + verifier-equality + memzero-on-failure, generateSalt uniqueness, generateDeviceSigningKeyPair (Ed25519 + BLAKE2b deviceId), generateKeyVerifier, X25519 ECDH roundtrip, deriveLinkingKeys, computeVerificationCode (deterministic, 6-digit, zero-padded), computeLinkingProof + computeKeyConfirm, getOrCreateSigningKeyPair, getOrDeriveVaultKey.

primitives.test.ts (118L) — generateFileKey uniqueness across 1000 calls + byte distribution, secureCleanup in-place zeroization.

Note to reviewer

The Argon2id inline snapshot is empty until first-run write. CI will need one -u pass to populate the hex — or the maintainer can run locally and commit the populated snapshot. Once populated, any parameter drift trips the test.

Test plan

• Run pnpm --filter desktop test --project main src/main/crypto/keys.test.ts -u once to populate Argon2id snapshot; commit the update
• pnpm --filter desktop test --project main src/main/crypto/keys.test.ts src/main/crypto/primitives.test.ts — 0 failures
• per-file coverage on both ≥ 95%
• pnpm typecheck:node && pnpm typecheck:web clean
• pnpm lint clean

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