Read autoload.files into ComposerAutoloadMap

[Firehed]

Slice S3.7a (#390). ComposerAutoloadMap gains the autoload.files set — the files
Composer loads wholesale rather than by name, and the only place a project's functions
and constants are locatable, since they have no name→file map (RFC 1 §3, Plan 0002 §3).

• autoloadFiles(): list<string>, read from vendor/composer/autoload_files.php.
  Composer keys that file by a content hash; the paths are taken as a plain list.
• Partitioned by vendor directory alongside the PSR-4/PSR-0/classmap splits, so each
  FilesystemBackend half sees its own files (RFC 1 §5.3).
• Non-string entries are discarded — generated data from a project we do not control.
• The fixture project declares two files entries, which also back S3.7b and S3.7d.

The accessor's consumer is ComposerSymbolLocator in S3.7d; it is public because that is
a different class, and caller-less until then because of the split.

Note: tests/Fixtures gained a files autoload section, so composer install needs
re-running there before the suite passes.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.58%. Comparing base (f40e791) to head (8c431ea).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff            @@
##               main     #392   +/-   ##
=========================================
  Coverage     98.57%   98.58%           
- Complexity     1751     1758    +7     
=========================================
  Files           116      116           
  Lines          4432     4451   +19     
=========================================
+ Hits           4369     4388   +19     
  Misses           63       63           

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.