Consolidate the duplicated, per-file Lit TemplateResult inspection helper
cluster into one shared, strictly-typed, type-guarded seam at
src/client/src/templateInspection.testSupport.ts, documented as the
testing-guide escape hatch. Migrate PiWebApp.clearQueue.test.ts to it as a
proof and delete its local copies. Tag not-yet-consumed public entry points
with @public so knip does not flag them until Slice B adds consumers.
Relay: testing-skill-compliance leg 1 (Finding 1).