fix: cache Playwright browsers instead of reinstalling them via apt

performance_smoke burned nearly all of its 15-minute budget before the
benchmark even started, twice in a row: validate.yml had no browser cache
at all, so every browser job paid for a full `playwright install
--with-deps` — apt work the ubuntu-latest image makes redundant, with
unbounded retries against an unreachable azure mirror on top. For a
measuring job that is worse than lost minutes: the timing window competes
with package installation on the same runner.

#175 fixed this for the review pipeline but deliberately left the flag
here, reasoning that a prerelease gate values predictability over
minutes. That reasoning was wrong — the flag is what made the gate
unpredictable.

Browsers are now cached per package-lock hash in smoke, golden,
performance_smoke and the full performance run; installation happens only
on a cache miss and no longer touches apt. performance_smoke keeps
headroom for a cold cache at 20 minutes. If the image ever drops a
required library, Chromium fails to launch with a clear missing-libraries
error; that is the moment to bring the flag back.

Issue: #206
User-Visible: no

.github/workflows/performance.yml

@@ -132,9 +132,17 @@ jobs:
       - name: Install candidate and baseline dependencies
         run: npm ci --prefix candidate && npm ci --prefix baseline
 
+      # То же, что в validate.yml: кэш браузеров, apt не трогаем (#206).
+      - name: Кэш браузеров Playwright
+        id: pw
+        uses: actions/cache@v6
+        with:
+          path: ~/.cache/ms-playwright
+          key: playwright-${{ runner.os }}-${{ hashFiles('candidate/package-lock.json') }}
       - name: Install pinned Chromium
+        if: steps.pw.outputs.cache-hit != 'true'
         working-directory: candidate
-        run: npx playwright install --with-deps chromium
+        run: npx playwright install chromium
 
       - name: Build both exact source trees
         run: |