docs: add testing guidance

[CEREBR4L]

Please describe what issue(s) this PR fixes.

Not tied to a specific issue. Testing isn't documented anywhere right now,
CONTRIBUTING.md doesn't mention it at all, so it's a guess for anyone
contributing whether a change needs a vitest spec, a playtest, or nothing.
I ran into that gap myself while trying to get more involved here, and
figured writing down what's actually going on (a few different tools for
different situations, most tests/playtests files aren't even tagged
@test on purpose, etc.) would give everyone, myself included, the same
shared understanding instead of each contributor re-deriving it on their own.

Summary

• Added TESTING.md: the three test tiers, where each kind of fix goes, and
  when it's fine to skip a test.
• Linked it from CONTRIBUTING.md, and added pnpm run test / test:vite
  to the pre-push checklist.

One thing I want to float rather than just decide: the doc suggests saying
why you're skipping a test, even if it's just "visual change, verified with
pnpm dev". Open to cutting that if it's not something you want to
normalize. As does quietly add the stupulation all PRs should at least
address testing.

Also aware #1116 is reworking testing too (Playwright, tests/e2e, a new PR
template with reviewer-side test checkboxes). This doc will need a follow-up
once that lands, or I'm happy folding it into that PR instead if you'd
rather not have two testing changes in flight at once.

• Changeloged