[Chore] Add mnemonic codec regression tests and a real npm test CI gate

[roomote-conceal-network]

​Created by Roomote. Follow up by mentioning @roomote, in the web UI, or in Discord.

What changed

Added a standalone C++ test harness for the mnemonic codec in cpp/Mnemonics — the first test coverage in this repository. tests/mnemonics_test.cpp compiles directly against Mnemonics.cpp, CRC32.cpp, and the word list with a plain host compiler (no React Native host app needed) via scripts/run-cpp-tests.sh, wired up as npm test.

The eight tests cover:

• Known-vector encode/decode round trip against a fixed key/phrase pair, plus round trips over multiple generated keys
• Mixed-case, fully upper-cased, and capitalized phrases decoding to the same private key as the lowercase phrase — the exact wallet-import bug fixed in [Fix] Wallet import fails or decodes the wrong key when mnemonic words are capitalized #4
• Checksum rejection when the checksum word or a data word is tampered with
• Wrong word count (24 and 26 words) rejection
• Unknown word rejection, including an unknown checksum word
• Checksum-word derivation and empty-phrase rejection

CI now runs npm test after the existing lint/format/check/cpp steps, so the tests gate merges. The npm run cpp / cpp:fix format scope was extended from cpp to cpp tests so the new test source is format-checked too.

Why this change was made

The critical mixed-case mnemonic decode fix from #4 (and polish in #5) merged with zero test coverage: no test script, no test files, and no CI test job, even though CLAUDE.md documented npm test as part of the quality gate. That doc/gate drift is closed by making the gate real and reconciling CLAUDE.md with what actually runs.

Impact

No runtime behavior changes — the codec itself is untouched. Refactors of the mnemonic codec are now guarded by regression tests; as a sensitivity check, the mixed-case test was verified to fail against a copy of Mnemonics.cpp with the #4 normalization fix reverted. The full local gate (npm run lint && npm run format && npm run check && npm run cpp && npm test) passes.

[roomote-conceal-network]

No code issues found. Reviewed the standalone mnemonic codec test harness, the npm test CI gate wiring, and the CLAUDE.md/package.json updates against the codec sources; ran the full local gate (npm run lint / format / check clean, and npm test — all 8 tests pass). See task

_{Reviewed c3b1a53}