Restore exclude/artifacts pattern support for force-include directories

[pctablet505]

Directories added via force-include used to have their contents filtered by the exclude and artifacts patterns (added in #297), but this got silently dropped when recurse_forced_files was split out from the shared file-selection logic in #299 (when only-include was introduced). Since then there's been no way to exclude specific files under a force-included directory.

This restores the old include_forced_path check for directory sources in force-include. To keep it correct, it uses a new forced_path_is_excluded helper rather than reusing path_is_excluded directly, since the latter also honors the "exclude everything" flag that's set when force-include is the sole file-selection mechanism for a build target — that flag is meant to blank out the default project file walk, not the forced files themselves.

Closes #2090
Closes #1989

Testing

• Added unit tests covering force-include + exclude, and force-include + exclude + artifacts (artifacts should still override exclude).
• Added an integration-level test using WheelBuilder for the case where force-include is the only file-selection option configured (the trickiest interaction, due to the "exclude everything" flag mentioned above).
• Ran the full tests/backend suite (939 passed) plus ruff check, ruff format --check, and mypy on the touched files.