e2e suite does not emit coverage; the coverage gate measures unit tests only

Formalizes the "(tracked follow-up)" note that lives only as a comment in .github/workflows/ci.yml (coverage-gate job, download step), raised again by review on #891.

The e2e job runs pnpm test (vitest with no --coverage), so tests/e2e/coverage/lcov.info is never written, the upload step's if-no-files-found: ignore silently skips, and the coverage >= 90% gate measures Rust unit coverage alone. #891 added per-leg artifact filenames so that the day e2e coverage turns on, the two matrix legs will not collapse into one file under merge-multiple: true — but turning it on is this issue.

Decisions to make when picking this up:

• whether TS harness coverage is the right signal at all (the e2e suite exercises the Rust binary; vitest coverage only measures the TS test/harness code), or whether the gate should stay unit-only and say so honestly;
• if it turns on: pnpm coverage in each matrix leg, and re-check the gate threshold since the combined denominator changes.

Read .github/workflows/ci.yml, especially the e2e job and coverage-gate download step, then review #891's per-leg artifact filenames. Compare the current pnpm test command with pnpm coverage in each matrix leg and decide which coverage signal the gate should represent. Done means the workflow and threshold accurately reflect that decision without silently ignoring expected coverage files.