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.

src/lsp/server.zig

@@ -1683,6 +1683,8 @@ pub const Server = struct {
             .kw_impl,
             .kw_inline,
             .kw_callconv,
+            .kw_extern,
+            .kw_export,
             .hash_run,
             .hash_import,
             .hash_insert,