[BUG] Honor workflow_dispatch filters in matrix include entries

[lukekarrys]

Problem

The benchmark matrix’s include: entries add build/next, build-cache/next, and run/run unconditionally. A filtered manual dispatch such as registry-only therefore still runs unrelated task-runner jobs and can wait roughly 40 minutes on build-cache.

That defeats the tight feedback loop added by #143 for partial dispatches.

Proposed direction

Generate the final matrix JSON in a setup job that honors the requested fixtures and variations. Add the special include pairs only when their variation is requested, or for scheduled/default runs.

Acceptance criteria

• A registry-only dispatch schedules only the requested registry fixture/variation pairs.
• Special build, build-cache, and run pairs are included for scheduled/default runs.
• Explicit dispatch filters can select each special variation without producing invalid fixture crosses.
• Matrix behavior is covered by a test/script or representative workflow-expression fixtures.
• The partial-run/deploy protection from ci: skip gh-pages deploy for partial benchmark runs #143 remains intact.

Source

Performance investigation: item 4 — matrix include pairs ignore dispatch filters