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/token.zig

@@ -43,6 +43,8 @@ pub const Tag = enum {
     kw_Self, // Self (in protocol declarations)
     kw_inline, // inline (compile-time if/for/while)
     kw_callconv, // callconv (calling convention annotation)
+    kw_extern, // extern (import: external linkage, C ABI, no body)
+    kw_export, // export (define + expose: external linkage, C ABI)
 
     // Symbols
     colon, // :
@@ -280,6 +282,8 @@ pub const keywords = std.StaticStringMap(Tag).initComptime(.{
     .{ "Self", .kw_Self },
     .{ "inline", .kw_inline },
     .{ "callconv", .kw_callconv },
+    .{ "extern", .kw_extern },
+    .{ "export", .kw_export },
 });
 
 pub fn getKeyword(bytes: []const u8) ?Tag {