Resolve SKiDL Timer library discovery in the design pipeline

[ReidSurmeier]

Context

The full suite has one expected failure: SKiDL cannot resolve the KiCad 9 Timer symbol library in the end-to-end design case. The wrapper correctly confines generated output, but the environment contract is incomplete.

Outcome

Make symbol-library discovery deterministic or return a more specific unavailable capability before generation.

Acceptance criteria

• add a focused failing test for the library-discovery boundary
• avoid machine-specific absolute paths
• make the end-to-end test pass or skip only on an explicitly detected missing capability
• verify no generated files escape the requested output directory