ci(e2e): shard the Electron Playwright suite per OS (#1700)

Run the sequential Electron E2E suite as three Playwright shards per OS
(one runner each) and summarize all shards of an OS in one follow-up job.
The semantic summary script accepts a directory of shard reports, merges
them and refuses to write when a shard is missing, duplicated or
malformed, or when an explicit input does not exist.

Slowest shard per OS in the final run: ubuntu 12.5 min (was 26),
macOS 13.7 min (was 34), Windows 24.5 min (was 35).

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

package.json

@@ -44,7 +44,7 @@
         "styles:inputs:test": "node --test tools/nx/check-stylesheet-inputs.test.mjs",
         "styles:inputs:check": "node tools/nx/check-stylesheet-inputs.mjs",
         "styles:inputs:validate": "pnpm run styles:inputs:test && pnpm run styles:inputs:check",
-        "coverage:tools:test": "node --test tools/coverage/coverage-integrity.test.mjs",
+        "coverage:tools:test": "node --test tools/coverage/coverage-integrity.test.mjs tools/coverage/e2e-shard-reports.test.mjs",
         "coverage:unit:ci": "node tools/coverage/run-tier-a-coverage.mjs",
         "coverage:merge": "node tools/coverage/merge-coverage.mjs",
         "coverage:health": "node tools/coverage/coverage-health.mjs",