CI: Collect e2e failure artifacts and generate merged HTML report

[ockham]

Add screenshot/trace artifact uploads per shard and a merge-artifacts job that combines them into a single downloadable HTML report, following the same pattern used in Gutenberg's end2end workflow.

AI Usage

Created with Claude Code (Opus 4.6) with the following prompt:

I'd like to change the current GHA workflow to collect artifacts from e2e -- specifically screenshots if a test fails, and to include them in the test run summary, much like Gutenberg does.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 52.97%. Comparing base (818c12a) to head (00708ed).
⚠️ Report is 2 commits behind head on trunk.

Additional details and impacted files

@@             Coverage Diff              @@
##              trunk     #374      +/-   ##
============================================
- Coverage     54.11%   52.97%   -1.15%     
  Complexity     4419     4419              
============================================
  Files           298      298              
  Lines         39448    39448              
============================================
- Hits          21348    20897     -451     
- Misses        18100    18551     +451     

Flag	Coverage Δ
e2e-js	45.71% <ø> (-0.02%)	⬇️
e2e-php	40.93% <ø> (-1.90%)	⬇️
javascript	15.07% <ø> (ø)
phpunit	29.96% <ø> (ø)

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