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>

CLAUDE.md

@@ -50,6 +50,43 @@ deploy-to-nixos production            # NixOS deploy
 
 Schema editing: use the IHP IDE at `localhost:8001` or edit `Application/Schema.sql` directly. Code generation via `localhost:8001/Generators`.
 
+## Compilation Layers
+
+IHP compiles ~180 Haskell modules as one GHC target. Module changes are incremental — only changed modules and their dependents recompile. **Never modify Layer 1 during error-fix loops** — a change to `Web/Types.hs` or `Generated/Types.hs` invalidates all 59 controllers and 120 views simultaneously.
+
+```
+Layer 1 — stable core (compile once):
+  build/Generated/Types.hs      IHP auto-generated from Schema.sql
+  Web/Types.hs                  controller type definitions
+
+Layer 2 — helpers (only touch if Layer 1 is clean):
+  Application/Helper/*.hs       12 helper modules
+
+Layer 3 — working surface (most errors live here):
+  Web/Controller/*.hs           59 controllers
+  Web/View/**/*.hs              120 views
+
+Layer 4 — wiring (fix last):
+  Web/FrontController.hs
+  Web/Routes.hs
+```
+
+**Compile tools** (run inside `devenv shell`):
+
+```bash
+scripts/compile-check          # full build via ghcid (all layers), log → /tmp/ihub-compile-errors.txt
+scripts/compile-check --bg     # same, background-friendly (no colour/title)
+scripts/compile-check-core     # Layer 1+2 only — verify clean base before touching Layer 3
+```
+
+**GHC object cache**: compiled `.o`/`.hi` files persist in `.devenv/` between restarts. After the first full build (20–60 min on constrained hardware), subsequent single-module rebuilds take 5–30 seconds. Cache location:
+
+```bash
+find /home/tegwick/inter-hub -name "*.hi" 2>/dev/null | head -5
+```
+
+**Error-fix discipline**: fix bottom-up (Layer 1 → 4). Fix one module at a time; wait for ghcid to reload before moving on. See `workplans/IHUB-WP-0016-build-infrastructure-and-error-loop.md` for the full SOP.
+
 ## Architecture
 
 ### Core Domain Model (Phase 1)