Cover namespace constant edge cases

[jbuncle]

Summary

• characterize richer namespace constant value cases including imported class constant fetches, imported constants, arrays, and magic constants
• add diff coverage for imported alias target changes inside namespace constants
• tighten the PHP 7 namespace-constant surface without broadening implementation scope

Validation

• udocker run -v /data/data/com.termux/files/home/projects/jbuncle/php-autosemver:/app -w /app composer2 sh -lc "find src bin tests -type f -name '*.php' -print0 | xargs -0 -n1 php -l && COVERAGE_MIN=90 COVERAGE_JSON=/tmp/coverage.json COVERAGE_SUMMARY=/tmp/coverage-summary.md phpdbg -qrr tests/coverage.php"
• coverage: 91.92% (910/990)