feat(WP-0016/C2-C3): close build infrastructure workplan

C2: document actual GHC cache behavior (-fbyte-code, no .o files, .hi
caching via -fwrite-interface) and correct CLAUDE.md cache claim.
C3: create IHUB-WP-0017 error-fix loop workplan (ralph-compatible,
5 tasks E1-E5, runs inside devenv shell).
WP-0016 status → done.

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

workplans/IHUB-WP-0016-build-infrastructure-and-error-loop.md

@@ -4,7 +4,7 @@ type: workplan
 title: "Build Infrastructure: Incremental Compilation and Autonomous Error-Fix Loop"
 domain: inter_hub
 repo: inter-hub
-status: open
+status: done
 owner: custodian
 topic_slug: inter_hub
 created: "2026-04-10"
@@ -71,6 +71,12 @@ skips the database startup, port binding, and service orchestration overhead.
 
 ### C1 — Create `scripts/compile-check` script ✓
 
+```task
+id: C1
+status: done
+priority: high
+```
+
 Also created `scripts/compile-check-core` and `.ghci-core` to compile Layer 1+2
 in isolation (no Main.hs, no Layer 3). Resolves the architecture gap where Layer
 2 correctness could not be verified independently.
@@ -103,19 +109,40 @@ This relies on `.ghci` (already present) which sets up `-j1` and loads the IHP c
 
 ### C2 — Verify GHC object cache persistence
 
-GHC stores compiled objects in IHP's `.devenv` or a project-local `dist-newstyle/`
-equivalent. Verify the cache survives between `devenv up` restarts:
-
-```bash
-# After first successful build, check cache location:
-find /home/tegwick/inter-hub -name "*.o" -newer Main.hs 2>/dev/null | head -5
+```task
+id: C2
+status: done
+priority: medium
 ```
 
-If the cache is cleared on each `devenv up` restart, add `IHP_BUILD_CACHE` or
-configure the Nix build dir to persist. Document the location here.
+**Finding (2026-04-10):** IHP's `applicationGhciConfig` sets `-fbyte-code`. GHCi compiles
+modules to bytecode in memory — there are **no persistent `.o` object files**. Each ghcid
+session recompiles all bytecode from source.
+
+Our `-fwrite-interface` flag (C4) writes `.hi` interface files alongside source modules.
+These survive session restarts and allow GHCi to skip re-type-checking unchanged modules.
+This saves some time on restarts but does NOT eliminate full bytecode recompilation.
+
+**Net result:** No persistent `.o` cache. `.hi` files provide partial speedup (skip
+type-checking). A single-module change still triggers full bytecode regeneration of that
+module and its dependents. For true persistent incremental builds, `-fobject-code` would
+be needed — not worth adding given IHP's design tradeoffs.
+
+**Action:** No configuration change needed. Expectation documented here and in CLAUDE.md.
 
 ### C3 — Autonomous error-fix loop (ralph-workplan)
 
+```task
+id: C3
+status: done
+priority: high
+```
+
+Delivered as `workplans/IHUB-WP-0017-error-fix-loop.md`. Run inside `devenv shell`:
+```bash
+/ralph-workplan workplans/IHUB-WP-0017-error-fix-loop.md
+```
+
 Configure a ralph-workplan loop:
 1. `scripts/compile-check --bg` runs ghcid in background, writing to `/tmp/ihub-compile-errors.txt`
 2. Monitor tool watches the log file for new error output
@@ -130,6 +157,12 @@ Configure a ralph-workplan loop:
 
 ### C4 — Add GHC `-fwrite-interface` and `-keep-going` flags to `.ghci` ✓
 
+```task
+id: C4
+status: done
+priority: high
+```
+
 ```
 -- Continue past errors in other modules (report all errors in one pass)
 :set -fkeep-going
@@ -142,6 +175,12 @@ stopping at the first failure — critical for batch error fixing.
 
 ### C5 — Document module surface constraints in CLAUDE.md ✓
 
+```task
+id: C5
+status: done
+priority: medium
+```
+
 Added "Compilation Layers" section to CLAUDE.md covering:
 - Never modify Layer 1 during error-fix loops (and why — cascade invalidation)
 - Layer breakdown with file counts