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.
bf6ef8370f