tokenizer: bring back to 100% coverage - #1603

Coverage claim is exact, measured with npm run cov:

So this closes precisely the global 99.86% funcs gap. The cause is confirmed by #1600's own body — the old fixture declared three arrows it never invoked, which is what cost the function score; unionX(0)(1)(2) allocates none.

I checked the type-erasure angle since it runs opposite to #1594/#1600 removing unreachable branches, and it holds up: the throw proof key is a first-class convention here (fjs/emergent_testing/module.f.mjs:65,91), and this throw is a real merge guard rather than a dead defensive branch. unionX is module-local, not exported, and const union = unionX re-narrows to _CreateToToken<T>, so every production call site (module.f.mjs:184) keeps its old type — no safety loss and nothing reachable from production.

Every dropped import is genuinely unreferenced; Reduce is used at lines 165 and 174. npx tsc exit 0; suite 2855 pass / 0 fail; CI green.

One nit: the description has no Changelog: section, which CONTRIBUTING.md:201 makes mandatory either way. Proof-only with no behavior change, so no changelog/unreleased/1603.md file is needed — just Changelog: none at the end of the body.