compiler-API: resume scaffolding for a fresh session

Add the COMPILER-API stream to CLAUDE.md's session-start router and a
`## ⏯ Resume` block to CHECKPOINT-COMPILER-API.md (next action = sub-step 2.2,
read order, build/verify, and the cross-arch snapshot-regen gotcha).

CLAUDE.md

@@ -339,21 +339,27 @@ Five active workstreams run in parallel — **IR** (the language compiler),
 overhaul, mem.sx + protocol expansion), **LANG** (user-facing language
 features — diagnostics renderer, heterogeneous variadic packs), and
 **ERR** (error handling: separate-channel `!` errors, `try` / `catch` /
-`or` / `onfail`, return traces). They touch mostly disjoint files;
-any can be advanced independently.
+`or` / `onfail`, return traces), and **COMPILER-API** (the comptime `compiler`
+library — `#library "compiler"` + `abi(.zig) extern compiler`; welded compiler
+types/functions that supersede the metatype `declare`/`define` `#builtin`s and
+the `#compiler` attribute). They touch mostly disjoint files; any can be
+advanced independently.
 
-1. Read all five checkpoints to see where each stream is paused:
+1. Read all checkpoints to see where each stream is paused:
    - `current/CHECKPOINT.md` — IR progress tracker.
    - `current/CHECKPOINT-FFI.md` — FFI progress tracker.
    - `current/CHECKPOINT-MEM.md` — MEM progress tracker + issues log.
    - `current/CHECKPOINT-LANG.md` — LANG progress tracker.
    - `current/CHECKPOINT-ERR.md` — ERR progress tracker.
+   - `current/CHECKPOINT-COMPILER-API.md` — COMPILER-API progress tracker
+     (has a `## ⏯ Resume` block; currently mid-Phase 2 on branch `reify`).
 2. Read the plan that corresponds to the stream the user wants to advance:
    - `current/PLAN.md` — IR implementation plan.
    - `current/PLAN-FFI.md` — FFI ceremony reduction plan.
    - `~/.claude/plans/tidy-doodling-cray.md` — MEM (mem.sx) implementation plan.
    - `current/PLAN-LANG.md` — LANG implementation plan.
    - `current/PLAN-ERR.md` — ERR implementation plan.
+   - `design/comptime-compiler-api.md` — COMPILER-API design-of-record + build order.
 3. Read `specs.md` if you need to understand language behavior.
 4. Pick up from the next incomplete step in the relevant `CHECKPOINT*.md`.
    If the user hasn't said which stream to work on, ask before picking.

current/CHECKPOINT-COMPILER-API.md

@@ -6,6 +6,22 @@ Companion to the design-of-record
 `declare`/`define`/`type_info` `#builtin`s and the `#compiler` struct attribute
 with ONE welded mechanism. Branch: `reify` (off `master`). Update after every step.
 
+## ⏯ Resume (fresh session)
+Phase 1 is COMPLETE and committed (`cd5b958`); Phase 2 (full byte-layout weld)
+just started. **Do sub-step 2.2 next** — make `src/backend/llvm/types.zig`'s
+`.@"struct"` case build a welded struct's LLVM type from `compiler_lib.computeWeldPlan`
+(offset-ordered field elements + `[N x i8]` padding) with a build-time
+`LLVMOffsetOfElement == plan offset` + `LLVMABISizeOfType == total_size` assertion;
+cache the plan per TypeId for the GEP sites. The plan math (sub-step 2.1) is done,
+pure, and unit-tested — see `computeWeldPlan` in `src/ir/compiler_lib.zig`. Full
+2.2–2.6 breakdown under **## Next step**. Read order: this file → the design doc →
+`src/ir/compiler_lib.zig`. Build/verify: `zig build && zig build test` (green now).
+
+> ⚠ Snapshot gotcha: `zig build test -Dupdate-goldens` on this aarch64 host clobbers
+> cross-arch examples' CI-captured `.stdout` (1228/1231/1639/1651/1657–1660) with
+> host-specific empties. After regenerating, revert those (`git checkout` / `rm`)
+> before committing — they are NOT part of this stream.
+
 ## Last completed step
 **Phase 2, sub-step 1 — the weld-plan layout math + `StructInfo` registered.**
 The de-risked core of the byte-layout-override ("GEP") engine, pure + unit-tested,