Fix makePathRelative using wrong base path when projectDirectory differs (#1608)

* Fix makePathRelative using wrong base path when projectDirectory differs (#1606)

`makePathRelative` used `project.basePath` to compute parent paths, while
`resolveGroupPath` used `projectDirectory ?? project.basePath`. When these
differ (e.g. via --project flag), intermediate group paths become inconsistent,
causing Xcode to resolve files to the wrong location.

Regression from #1596 which added the `makePathRelative` call in the
`createIntermediateGroups` code path.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

* Extract basePath computed property in SourceGenerator

Consolidates `projectDirectory ?? project.basePath` into a single
`basePath` property to prevent future mismatches between path resolution
call sites.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>