feat(ffi-linkage): extern/export parser+AST plumbing, unconsumed (Phase 0.1)

Add ast.ExternExportModifier { none, extern_, export_ } beside
CallingConvention; FnDecl.extern_export and VarDecl.is_extern/extern_name
fields (all defaulting to absent); and Parser.parseOptionalExternExport()
mirroring parseOptionalCallConv.

None of this is consumed by a decl path yet — no user-facing behavior
change, corpus diff empty. Two inline parser unit tests pin the helper's
keyword mapping and the field defaults. Phase 1.0 wires the helper into
the fn-decl path. lock commit.

current/CHECKPOINT-EXTERN-EXPORT.md

@@ -5,13 +5,14 @@ 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 0.0** (lock) — added `kw_extern`/`kw_export` to `token.Tag` + the
-`keywords` `StaticStringMap` (beside `kw_callconv`, `token.zig:45,282`); classified
-both as `ST.keyword` in the LSP `classifyToken` exhaustive switch
-(`lsp/server.zig`); added `test "lex linkage keywords"` in `lexer.zig`. Suite green
-(441 unit / 633 corpus, 0 fail). `extern`/`export` are now reserved words; corpus
-sweep confirmed no `.sx` identifier used either (only comments + the unpinned
-`issues/0030-*` repro, which doesn't run in the suite).
+**Phase 0.1** (lock) — added `ast.ExternExportModifier = enum { none, extern_,
+export_ }` (beside `CallingConvention`), `FnDecl.extern_export` (default `.none`),
+`VarDecl.is_extern`/`extern_name` (defaults absent), and
+`parser.parseOptionalExternExport()` (mirrors `parseOptionalCallConv`, just below it
+at `parser.zig:~3683`). **Helper + fields defined but NOT consumed by any decl path**
+— no user-facing behavior change, corpus diff empty. Two inline parser unit tests
+(`parseOptionalExternExport recognizes linkage keywords`, `extern/export AST fields
+default to absent`). Suite green (443 unit [+2] / 633 corpus, 0 fail).
 
 ## Current state
 Syntax decided + ratified: bare `extern`/`export`, postfix in the `callconv(.c)`
@@ -20,15 +21,17 @@ slot, `extern ⇒ callconv(.c)`, library separate. Touch-points mapped — token
 `decl.zig:1123,387,2110,2382,2514`; IR/emit already capable (no codegen change).
 Export gap = 4 lowering conditions. Part B `foreign` footprint to purge: 643 lines /
 ~57 identifiers in `src/` + 28 doc lines. End-state invariant: **zero `foreign`** in
-the live tree (Phase 9.4 gate). Tokens exist (0.0); parser/AST not yet plumbed.
+the live tree (Phase 9.4 gate). **Phase 0 done**: tokens (0.0) + AST/parser plumbing
+(0.1) exist, unconsumed. Phase 1 wires `extern` into the fn/global decl paths.
 
 ## Next step
-**Phase 0.1** (lock) — `parseOptionalExternExport()` (mirror `parseOptionalCallConv`,
-`parser.zig:3669`) + `ast.ExternExportModifier` enum + `FnDecl.extern_export` +
-`VarDecl.is_extern`/`extern_name` fields; **parsed but not yet consumed**; unit AST
-test. Then Phase 1 (`extern` working: 1.0 xfail accept postfix, 1.1 green lower via
-`declareExtern`, 1.2 green rename + extern-global). See the plan's **"Kickoff
-prompt"**. Stop at end of Phase 1.
+**Phase 1.0** (xfail) — accept postfix `extern` after the callconv slot in the
+fn-decl path (`parser.zig:1950`: call `parseOptionalExternExport()`, store on
+`FnDecl.extern_export`); add `examples/12xx-ffi-extern-fn.sx` that extern-binds a libc
+symbol — **red** (parses, but lowering not wired yet). Then 1.1 green (lower via
+`declareExtern`: `is_extern`, `.external`, `callconv(.c)`, no ctx — anchors
+`decl.zig:1123,387,2110,2113`), 1.2 green (`extern "csym"` rename + extern-global
+`g : T extern;`, `parser.zig:425`). Stop at end of Phase 1.
 
 ## Open decisions
 Part A ratified (bare / postfix / `⇒ callconv(.c)` / lib-separate). Part B (confirm
@@ -41,6 +44,9 @@ historical carve-out — keep `issues/*.md` provenance, gate the live tree only.
 - (0.0) Added `kw_extern`/`kw_export` tokens + keyword-map entries + LSP keyword
   classification + `lex linkage keywords` test. Suite green; no identifier collisions
   in the corpus. `lock` commit.
+- (0.1) Added `ast.ExternExportModifier` + `FnDecl.extern_export` +
+  `VarDecl.is_extern`/`extern_name` + `parseOptionalExternExport()` (unconsumed) + 2
+  parser unit tests. Suite green (443/633). `lock` commit.
 
 ## Known issues
 None yet.