Fix Windows path handling and runtime isolation

[levnikolaevich]

Summary

Fix four Windows/runtime issues discovered while validating scip-php on a real Laravel project:

• trim Composer autoload paths with both / and \\
• discover the indexer's own vendor/ correctly when scip-php is installed as a dependency under vendor/davidrjenni/scip-php
• unregister the target project's autoloader when scip-php runs outside the target vendor/
• normalize emitted relative_path values so Windows artifacts stay project-relative

Closes #814.

Validation

• vendor/bin/phpunit --no-coverage tests/Composer/ComposerTest.php
• vendor/bin/phpunit --no-coverage tests/Indexer/IndexerTest.php
• live repro on a real Laravel project on Windows now produces a non-empty .scip artifact with project-relative document paths
• downstream validation in hex-graph-mcp successfully imported the generated PHP SCIP artifact back into the native graph and materialized scip_import edges