feat(ffi-linkage): add kw_extern/kw_export tokens (Phase 0.0)

Lex 'extern' and 'export' as keywords beside 'callconv': new token.Tag
variants + keywords StaticStringMap entries + LSP semantic-token keyword
classification. Adds a 'lex linkage keywords' unit test.

Tokens only — parser/AST plumbing and lowering land in later phases.
Corpus sweep confirmed no .sx identifier collides with the new reserved
words. lock commit per the cadence rule.

current/CHECKPOINT-EXTERN-EXPORT.md

@@ -5,8 +5,13 @@ 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
-None — plan authored, not yet started. (Merged the former EXTERN-EXPORT +
-FOREIGN-MIGRATION plans into this single plan.)
+**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).
 
 ## Current state
 Syntax decided + ratified: bare `extern`/`export`, postfix in the `callconv(.c)`
@@ -15,12 +20,15 @@ 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). No code written.
+the live tree (Phase 9.4 gate). Tokens exist (0.0); parser/AST not yet plumbed.
 
 ## Next step
-**Phase 0.0** — add `kw_extern`/`kw_export` tokens + keyword-map entries (beside
-`kw_callconv`) + unit lex test. Then 0.1 (parser plumbing, unconsumed), then Phase 1
-(`extern` working). See the plan's **"Kickoff prompt"** section.
+**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.
 
 ## Open decisions
 Part A ratified (bare / postfix / `⇒ callconv(.c)` / lib-separate). Part B (confirm
@@ -30,6 +38,9 @@ historical carve-out — keep `issues/*.md` provenance, gate the live tree only.
 ## Log
 - (init) Plan written; FFI-linkage stream opened.
 - (merge) Folded FOREIGN-MIGRATION in as Part B; deleted the split plan + checkpoint.
+- (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.
 
 ## Known issues
 None yet.