fix(check): keep auditing when a motion spec is partly unusable (#2805)

Combines two fixes to the motion sidecar path:

- An ambiguous selector no longer aborts the whole spec. It is reported
  as a finding, the assertions that depend on it are skipped and named,
  and every other assertion still evaluates. `assertionTargets` is now
  exhaustive over the assertion kinds so a new kind cannot silently fall
  back to an arbitrary first match.
- A sidecar that will not parse is reported as a spec finding instead of
  ending the run, so the composition is still audited.