CLAUDE.md: add InternPool gap porting workflow

Document the workflow for closing the IP entry gap between the Zig
compiler and C sema.  Starting with neghf2.zig, corpus tests require
the C sema to create ~878 module-level IP entries (struct types,
ptr_nav, enum types, etc.) matching the Zig compiler's output.

The workflow describes how to dump the Zig compiler's IP state, compare
with the C sema, port module-system functions (createFileRootStruct,
scanNamespace, etc.), and iterate.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

stage0/CLAUDE.md

@@ -43,6 +43,70 @@ take shortcuts. Instead, decompose:
 5. Repeat until enough pieces are in place for the corpus test to pass.
 6. Return to [The loop](#the-loop).
 
+## Closing the InternPool gap (neghf2 and beyond)
+
+Starting with `neghf2.zig`, corpus tests require the C sema to produce
+InternPool entries that match the Zig compiler's module-level analysis.
+The Zig compiler creates ~878 IP entries before analyzing `__neghf2`'s
+function body (struct types for modules, pointer-to-Nav entries, enum
+types/values, function types, etc.). The C sema must create the same
+entries so that IP indices in the function body AIR match.
+
+### Background
+
+- Nav entries (declarations) are stored in a **separate** list from IP
+  Items — they do NOT consume IP indices. But `ptr_nav` entries (pointers
+  TO Navs) DO consume IP indices.
+- The Zig compiler creates IP entries through `createFileRootStruct`
+  (in `src/Zcu/PerThread.zig`) and `scanNamespace`. These must be
+  ported to C.
+- Source of truth for the IP entry sequence: add a temporary debug print
+  in `src/Zcu/PerThread.zig` at the `verbose_air_callback` call site
+  (around line 4478) to dump `ip.locals[0].shared.items` for the
+  function being debugged. Build with `zig build test-zig0` to compile
+  a new `air_gen`, then run it directly on the target file. **Always
+  revert** the debug print before committing.
+
+### The module-system porting loop
+
+1. **Dump the Zig IP.** Temporarily add debug output in
+   `src/Zcu/PerThread.zig` at the `verbose_air_callback` site.
+   Rebuild, run the `air_gen` binary directly on the target corpus
+   file (e.g. `lib/compiler_rt/neghf2.zig`), capture the IP entries.
+   Revert the debug print.
+2. **Compare.** Run `zig build test-zig0` with `num_passing` bumped.
+   Note the mismatch: `a=0x???[ip] b=0x???[ip]`. The gap `a − b` is
+   the number of missing IP entries.
+3. **Port the next batch.** Identify what the Zig compiler creates for
+   the next ~10 IP entries (struct types, ptr_nav, enum types, etc.).
+   Port the corresponding logic from `src/Zcu/PerThread.zig` and
+   `src/Sema.zig` into `stage0/sema.c`.  Key functions to port:
+   - `createFileRootStruct` → creates `type_struct` IP entry for a
+     module's root.
+   - `scanNamespace` → iterates declarations, creates `ptr_nav` entries
+     for each Nav.
+   - `getStructType` / `getEnumType` → creates type entries in IP.
+   - `ensureFileAnalyzed` → recursively processes imported modules.
+   - `zirExport` → forces resolution of exported declarations.
+4. **Test.** `zig build test-zig0` — the gap should shrink.
+5. **Clean up & commit** (see [Cleaning Up](#cleaning-up)). Keep
+   `num_passing` at whatever value passes; don't bump it until the gap
+   reaches zero.
+6. **Repeat** until the gap is zero and `num_passing` can be incremented.
+
+### Important constraints
+
+- Do NOT hardcode IP entries from a dump. The entries must be computed
+  from the ZIR, matching the Zig compiler's processing.
+- Do NOT include generated `.c` files. All logic belongs in `sema.c`.
+- Entry ORDER matters. The C sema must create entries in the same
+  order as the Zig compiler. Follow the Zig compiler's processing
+  sequence (struct type → scan declarations → process imports →
+  resolve comptime blocks).
+- Deduplication matters. If the function body interns a value that was
+  already created during module-level analysis, `ipIntern` must return
+  the existing index (not create a duplicate).
+
 ## AIR comparison exceptions
 
 C and Zig AIR must match byte-by-byte except: