Separate native linking from C compilation (#278)

* Separate native linking from C compilation

 - Split the helper-object runtime-pack path into explicit link planning and platform linking.
 - Reserve SCRIPTC_LINKER for object-only executable links while retaining a deprecated SCRIPTC_CC compatibility path.
 - Add no-legacy-C coverage and document the linker/SDK boundary.

* Preserve runtime pack link errors

* Fix linker cache invalidation and warnings

* Fix C executable cache routing

- Keep explicit C builds on compiler-backed cache identities.
- Cover linker-only environment changes across exact C repeats.