docs(ffi-linkage): checkpoint — Phase 5.1 done; PHASE 5 COMPLETE, next Phase 6

current/CHECKPOINT-EXTERN-EXPORT.md

@@ -5,7 +5,20 @@ Companion to `current/PLAN-EXTERN-EXPORT.md` — one merged plan: **Part A** add
 every commit, one step at a time per the cadence rule.
 
 ## Last completed step
-**Phase 5.0 — fn-decl `#foreign` body-marker FLIP** (`refactor` commit `6b94bb6`).
+**Phase 5.1 — annotate A→B gate post-flip + add fn-rename case** (`test` commit
+`93e7b6f`). **PHASE 5 COMPLETE → PART B Phase 5 done.** The A→B gate
+(`lower.test.zig`) already asserted `#foreign` ≡ `extern`/`export` byte-identical IR
+for fn / global / Obj-C class; post-Phase-5.0 the fn-decl + data-global paths build
+the SAME extern-named AST, so cases 1/2 are now STRUCTURALLY identical (guaranteed by
+construction, not coincidence). Annotated the gate header to record this and keep it
+as a regression tripwire (catches a future reader re-diverging the spellings, or a
+revert of the flip); case 3 (runtime class) stays behaviorally — not structurally —
+equal via the single `is_foreign_eff` field. Added a fn-rename case (case 2b,
+`extern_name` axis: `c_abs` → `"abs"`) to broaden coverage beyond bare import
+(verified IR-identical via `sx ir` probe before adding). Test-only, no snapshot churn.
+Suite green (647 corpus / 444 unit, 0 failed).
+
+### Prior: Phase 5.0 — fn-decl `#foreign` body-marker FLIP (`refactor` commit `6b94bb6`).
 **PHASE 5.0 PARSER ROUTING COMPLETE.** The fn-body `#foreign [LIB] ["csym"]` marker
 now builds the SAME extern AST postfix `extern` produces (`extern_export = .extern_`
 + `extern_lib`/`extern_name` + empty-block body) instead of a `foreign_expr` body.
@@ -147,15 +160,19 @@ AOT), 1227 (export fn rename, AOT), 1348 (objc extern class), 1349 (objc export
 (jni extern class), 1174/1175 (interplay diagnostics).
 
 ## Next step
-**PART B — Phase 5.1: lock `#foreign` ≡ `extern` IR, then Phases 6–7 (migrate
-stdlib + examples).** Phase 5.0 parser routing is COMPLETE.
+**PART B — Phase 6 (migrate stdlib `#foreign`→`extern`, empty snapshot diff per
+batch).** Phase 5 (parser routing 5.0 + gate lock 5.1) is COMPLETE.
 
-- **Phase 5.1** (`lock`/`test`): the A→B gate (`lower.test.zig`) already asserts
-  `#foreign` ≡ `extern`/`export` byte-identical IR for fn / global / Obj-C class, and
-  it's now TRIVIALLY true for the fn path (both build the same AST). Decide whether 5.1
-  needs anything beyond the existing gate — likely just confirm/annotate it covers the
-  post-flip world (the fn path no longer has two distinct shapes to compare). Could fold
-  into a doc note rather than a new test.
+- **Phase 6** (`refactor` batches, one commit per batch, snapshot diff must be EMPTY):
+  rewrite `library/` `#foreign`→`extern` (fns/globals), `#foreign #objc_class`→
+  `#objc_class … extern`, defined classes → `… export`. Pure SOURCE rename — the AST is
+  already unified, so IR/output must be byte-identical per batch. A non-empty diff ⇒ a
+  reader still branches on `foreign_expr` structurally → STOP and coalesce it first
+  (don't regen). Batches (from PLAN-EXTERN-EXPORT Phase 6): 6.1 `library/vendors/sqlite/`
+  (~98 sites), 6.2 `library/modules/platform/` (uikit/android/android_jni/sdl3, ~95),
+  6.3 `library/modules/std/` (socket/thread/fs/time/process, ~60), 6.4
+  `library/modules/ffi/` (objc/raylib/objc_block, ~50), 6.5 remaining `library/`+
+  `vendors/`. Then **Phase 7** migrates examples + issues the same way.
 - **Phases 6–7** (`refactor` batches, empty snapshot diff per batch): migrate the
   stdlib + examples from `#foreign` spelling to `extern`. Because the AST is already
   unified, this is a pure SOURCE rename (`… #foreign LIB "sym";` → `… extern LIB "sym";`
@@ -278,6 +295,12 @@ Part A ratified (bare / postfix / `⇒ callconv(.c)` / lib-separate). Part B:
   deleted at cutover). Affects only diagnostic wording — IR/behavior identical either way.
 
 ## Log
+- (5.1 gate annotate) **PHASE 5 COMPLETE.** Annotated the A→B gate header
+  (`lower.test.zig`) to record that post-Phase-5.0 the fn/global `#foreign` paths
+  build the same extern-named AST → cases 1/2 are structurally (not coincidentally)
+  identical; the gate stays as a regression tripwire. Added fn-rename case 2b
+  (`c_abs` → `"abs"`, `extern_name` axis), IR-identical per a `sx ir` probe.
+  Test-only, no snapshot churn. Suite green (647/444). `test` `93e7b6f`.
 - (5.0 fn-body flip) **PHASE 5.0 PARSER ROUTING COMPLETE.** Flipped the fn-body
   `#foreign` parser arm (`parser.zig:~2062`) onto the extern AST (empty-block body +
   `extern_export = .extern_` + extern_lib/extern_name); `extern_export` made `var` so