feat(WP-0016/C5): compilation layers docs and -fwrite-interface flag

- CLAUDE.md: add "Compilation Layers" section (C5) — layer map,
  compile-check tool usage, cache location, error-fix discipline
- .ghci: add -fwrite-interface so interface files are cached even
  on partial-success builds (completes C4 spec)
- WP-0016: mark C5 done; annotate C4 ✓ note in workplan

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

.ghci

@@ -7,4 +7,6 @@
 -- Report errors from all modules in one pass (don't stop at first failure).
 -- Critical for batch error fixing — see IHUB-WP-0016 C4.
 :set -fkeep-going
+-- Write interface files even on partial success so cached modules are not re-compiled.
+:set -fwrite-interface
 import IHP.Prelude