chore: split uneval into an internal analyze/render emitter

[elliott-with-the-longest-name-on-github]

Splits uneval's internals into two steps, so unevalStream (#218) can reuse the emitter without growing uneval a pile of special cases. The split also gives the stream a clean point between walk and render: once the walk has run, it knows whether the value contained any promises, and a head without them is just analysis.render(), identical to uneval. Nothing public changes and the output is byte-for-byte the same.

const analysis = analyze(value, { replace, known, enter, reserved }); // walk + name allocation
analysis.render({ reference, retain, primitive }); // build the string, at most once

uneval(value, replacer) is now just analyze(value, { replace }).render(). Basically, the walk stays exactly where it was, and everything after walk(value) moves into render. Render reuses the walk's state (seen, names), so calling it twice throws instead of quietly producing garbage. known returns a token (the stream uses its own Location) and render passes that same token to reference, so there's no second lookup. The rule for which primitives are worth a slot (strings ≥128 chars, bigints with ≥128-char literals) now lives in the emitter, so primitive only gets called for those.

Hooks are only checked where they were before. A plain uneval call costs one extra object (the Analysis), and the no-hook path reuses a shared frozen {} instead of allocating hook objects. Typical cases in pnpm bench:compare fall inside run-to-run noise. On the micro end (small object, 100k calls of a 3-key object) it's ~3% slower, about the same as the hooks version this replaces. That's the per-call closure setup, not per-node work.

There's also a separate commit fixing the benchmark imports, which have pointed at ../../index.js since #209 moved it to src/. Without it, bench:compare doesn't run on main at all.

test/uneval-hooks.test.js covers each hook. It also checks that analyze(v).render() matches uneval(v) across a spread of fixtures, that a second render throws, that tokens make the round trip, and that short primitives never reach primitive.

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