ci: parse xcresult bundles in one place

[datlechin]

Why

summarize-xcresult.sh and verify.sh each parsed result bundles, and they drifted. The CI summary printed 0-based lines, one line high, listed "Testing cancelled because the build failed." as a build error, and counted a "System Failures" runner failure as a failed test, which hid "executed no test".

Change

• scripts/lib/xcresult-cases.py (stdlib, Python 3.9) reads bundles and xunit files for both: tally and errors for verify.sh, summary for CI.
• summarize-xcresult.sh calls it; platform-apple.sh drops its jq, awk and xmllint parsers.
• CI summary: 1-based lines, no "Testing cancelled", runner failures listed apart and not counted as executed tests.
• verify.sh now says to rerun generate-project.sh when a base xcconfig is missing.

Verified

scripts/lib/test_xcresult_cases.py, now run by Repo Hygiene. Old and new parsers agree on real bundles (retries, nested and parameterized Swift Testing, a failed build, a CI runner failure) apart from the fixes above. verify.sh test SQLEscapingTests SpatialProjectionTests passes through it.

Not in this PR

• detect-changed-paths.sh's oracle suite does not watch scripts/lib/xcresult-cases.py yet.