tests: raise sdk-node coverage from 62% to 93%

[TeoSlayer]

Summary

Adds five new test files and a vitest.config.ts that scopes v8 coverage to src/. Raises measured coverage on the published SDK surface from 62.03% to 92.93% (v8 statements/lines) with 173 tests across 7 files, all green.

File	Before	After
cli.ts	0%	100%
client.ts	67.85%	99.48%
ffi.ts	40%	86.81%
runtime.ts	74.01%	88.98%
index.ts	(excluded re-export shim)	—
overall	62.03%	92.93%

Source: @vitest/coverage-v8@^3.0.0 (matches the pin already used in .github/workflows/ci.yml).

What's new

• vitest.config.ts — scopes coverage to src/**/*.ts, excludes the re-export shim, adds json-summary reporter.
• package.json — new test:coverage script; coverage-v8 promoted to a dev-dep (CI already installs it inline, this just keeps local installs consistent).
• tests/client_extra.test.ts (20 tests) — covers sendMessage, sendFile, publishEvent, subscribeEvent, hostname resolution, library-singleton helpers, and the ACK-read fallback branches in sendMessage/sendFile. Uses an in-memory FFI fake so the wire frame layout ([type][len][payload], event frames, file-transfer name header) is exercised end-to-end.
• tests/ffi_extra.test.ts (12 tests) — parseJSON / checkErr / unwrapHandleErr edge cases plus findLibrary's PILOT_LIB_PATH override and the runtimeLibraryPath fallback that exercises the seeder.
• tests/runtime_extra.test.ts (10 tests) — isDaemonLive against a real ephemeral UNIX-socket server (live + missing + dead-file + malformed-config), seeder daemon-skip behaviour, version fallback to package.json, stale-lock reclamation, ensureDirWritable error message.
• tests/cli.test.ts (16 tests) — covers all four bin-stubs shims using a hoisted spawnSync mock; verifies success / non-zero / default-1 / launch-error paths. process.exit is replaced with a thrower so the test runner survives.
• tests/index.test.ts (4 tests) — smoke-checks the public re-export surface.

What's hard to test without a live daemon

• loadLibrary()'s koffi-loaded function bodies (the four Pilot* wrappers that allocate buffers via koffi.decode) — these need the actual libpilot.{so|dylib} on disk. Currently 86.8% covered because findLibrary discovers a bundled lib when one is present in ~/.pilot/bin/ from a previous SDK install; on a fresh CI runner without a bundled binary the loader path will be skipped and coverage will drop to ~40%. Recommend an integration test that builds and links libpilot in CI, or an FFI replay/stub harness if you want to stay unit-test-only.
• cli.ts actually executing the seeded binaries — current tests mock spawnSync, so we verify the contract but not the real process launch.
• probeDaemonLiveSync — uses nc -z -U synchronously. The probe is environment-dependent (see "Bugs noticed" below), so the test asserts a tolerant invariant ("either upgrade or daemon-skip") rather than a single fixed outcome.

Bugs noticed (NOT fixed in this PR)

1. probeDaemonLiveSync is unreliable — BSD nc -z -U <sock> on macOS returns status 1 even when the socket is connectable, so the seeder may overwrite pilot-daemon while it is actually running. The async isDaemonLive (which uses a real net.Socket.connect) is correct; the sync probe should use the same primitive via spawnSync(node, ['-e', '...']) or expose an async seeder API.
2. probeDaemonLiveSync require('node:child_process') fails in pure ESM — In an ESM module, the require call inside probeDaemonLiveSync throws, falling through to the conservative existsSync(sockPath) path. Result: ANY socket file at the configured path is treated as a live daemon and pilot-daemon is silently skipped. This is the same root cause flagged by iter-3 as a HIGH bug.
3. PILOT_LIB_PATH allows arbitrary .so/.dylib load with no signature / checksum / origin check. Documented for posterity; expected behaviour for an FFI escape hatch, but worth a warning in the README.

Test plan

• npm test — 173 tests pass, no warnings
• npm run test:coverage — 92.93% statements / 92.93% lines / 85.80% branches / 85.96% functions on src/
• Three repeated runs of npm test for flakiness; all stable
• CI run on Linux x64 (will likely show lower ffi.ts coverage if no bundled libpilot — see note above)
• Optional: confirm Codecov upload picks up the new reporter outputs