Improve native dynlib build integration

[elcritch]

Summary

• move native dynlib build configuration into reusable figdraw/build/tasks
• resolve Figdraw paths from the source location and inherit the active Nim compiler search paths, keeping nested builds independent of the caller working directory and package-manager layout
• expose staged-artifact checks so downstream config.nims files can build only when needed while the build_dynlib task always forces a rebuild
• clean the backend-specific native cache before forced builds so orphaned BIF files from another compiler or checkout cannot create duplicate ABI types
• extend the dynlib facade with managed image handles and renderer atlas/resource operations

Validation

• nim build_dynlib, including with a stale conflicting Rune BIF injected into the producer cache
• nim check -d:useNativeDynlib src/merenda/nimkit.nim, with staged dylib and ABI timestamps unchanged
• atlas-run tests --only-errors tnimkit -- -d:useNativeDynlib in Merenda
• atlas-run tests --compile-only --only-errors examples/all_compile.nim -- -d:useNativeDynlib in Merenda