[rpgmem]

Closes #2058

Problem

A CI run takes about 20 minutes. Coverage shards 2–4 take 14 to 20 minutes each. The two plain PHPUnit jobs each run the whole suite in one process, in 10 to 17 minutes. PHPUnit (PHP 8.3) repeats what the coverage shards already do: they run the same suite on PHP 8.3. Measurements are in #2058.

Change

• Coverage: 4 → 6 shards.
• PHP 8.4: 3 shards (PHPUnit 8.4 shard N) instead of one process.
• PHP 8.3: the plain run is removed.
• Required check names stay the same: PHPUnit (PHP 8.3) and PHPUnit (PHP 8.4) are now gate jobs. They report the result of their shards (the coverage shards for 8.3) and run nothing themselves, like Coverage (PHP 8.3) already did. Branch protection needs no change.
• Integration suite included: shard-tests.php now takes both suites phpunit.xml.dist declares, so tests/Integration runs in the shards. The coverage shards used to skip it.
• CLAUDE.md: the required-check note now names the gates and their shards.

Left alone: the LPT weighting. Locally the 4 shards were balanced (14–16 min with 4 in parallel), and the spread seen on CI is runner variance.

Tests

• Every new shard passes locally, as a check for order dependence: 6 shards and 3 shards, both suites, 693 files, each file in exactly one shard per partition.
• The workflow parses, and every job's needs resolves.

Risk

CI only. The new partition places files in different neighbours, so a latent order dependence could surface. The local run of every shard is there to catch it before CI does.

Checklist

• Base is develop; no version bump
• CHANGELOG.md — bullet at the end of Changed
• Documentation topic — not applicable
• Strings — none
• Minified assets — none touched
• Fast checks run locally

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