fix: Tia coverage cache recorded on another machine empties the merged report and passes --min

[AlessioGiacobbe]

Fixes #1796.

The bug

A TIA baseline recorded on one machine (e.g. GitHub Actions) ships its coverage cache with the recording machine's absolute paths serialized inside. When another machine downloads it via pest()->tia()->baselined() and runs --tia --coverage, CoverageMerger merges that cache with the local run's coverage, producing a data set with two disjoint path roots. php-code-coverage's report builder silently skips every file whose reconstructed path fails is_file(), so the final report has zero file nodes: Pest prints an empty per-file table with a bogus Total: 100.0 %, and --coverage --min=X passes regardless of the real coverage — the check fails open with no warning.

The fix

Make the cache path-portable and refuse to merge what cannot be mapped:

• Write side (serializeRelativeToProjectRoot()): the cache now stores file paths relative to the project root (canonical / separators), so a baseline recorded on CI stays meaningful on any machine that downloads it.
• Read side (readCache() / rebaseOntoProjectRoot()): relative entries are mapped onto the local project root before the strip + merge. Absolute entries under the local root are kept, preserving caches written by previous versions on the same machine. Absolute entries of another machine make the cache unusable — it is discarded and the current run seeds a fresh (portable) one, so the printed report falls back to correct current-run data instead of an empty table that satisfies --min.
• An unreadable cache (gzip or unserialize failure) now also reseeds from the current run; previously it was deleted and the run produced no cache at all.

Tests

tests/Unit/Plugins/Tia/CoverageMergerPortability.php (new):

• a cache recorded under another root is discarded, the marker is consumed, and the current run becomes a relative-path cache (fails on 5.x by keeping the foreign-path merge);
• relative cached entries are rebased onto the local root before merging, and the rewritten cache is relative again;
• a same-machine cache with absolute paths still merges (back-compat).

Note: this touches the same applyIfMarked() flow as #1792 — the two fixes are independent but will conflict textually; happy to rebase whichever lands second.

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