types/patricia_trie: fold the stack instead of indexing it - #1582

Implements fjs/types/patricia_trie/todo/declarative-stack-fold.md (deleted here).

end

It was a right fold written as a descending index loop with a let and a mutating destructuring assignment:

end: ([storage, stack]) => {
    if (stack.length === 0) { return [undefined, storage] }
    let h = stack[stack.length - 1][1]
    for (let i = stack.length - 2; i >= 0; i--) {
        const lHash = stack[i][1];
        [h, storage] = create(lHash, h, storage)
    }
    return [h, storage]
}

splitLast supplies the seed and the rest together, so the empty stack becomes the null branch and the separate length guard disappears:

end: ([storage, stack]) => {
    const split = splitLast(stack)
    if (split === null) { return [undefined, storage] }
    const [rest, [, lastHash]] = split
    return rest.reduceRight(([h, s], [, lHash]) => create(lHash, h, s), [lastHash, storage])
}

No let, no index arithmetic, no mutation.

push

The TODO's call — that push's loop is genuine carry propagation and can stay imperative, but should split rather than index — is right: each merge decides whether the next one happens, so it is sequential in a way end is not. What it does not need is stack[stack.length - 1], stack[stack.length - 2] and stack.slice(0, -2), which are now two splitLast calls. Either split answering null is exactly where the old stack.length >= 2 guard stopped, so the guard is not lost, just expressed by the same operation that reads the elements.

Verification

This builds content-addressed trees, so identical structure matters more than a green suite:

• npx tsc clean.
• fjs test: 2797 pass, 0 fail.
• npm run cov: fjs/types/patricia_trie/module.f.mjs at 100% lines/branches/functions.
• Differential check against main. 246 sorted leaf sequences — every length 0 through 40, six deterministic pseudo-random 64-bit sets each — pushed one at a time and drained with end. Compared the root identity, the node count, and every (left, right, hash) triple in storage: 406 KB of output, byte-identical. That covers the empty stack, the single-leaf stack (where end returns the seed without folding), and deep carry chains.

One note on the splitLast dependency: its elements here are Candidate<T> tuples, never null, so the nullish-element defect filed as fjs/types/array/todo/split-accessors-nullish-element.md cannot bite this caller.

Changelog:

• types/patricia_trie: end is a right fold over the candidate stack and push splits instead of indexing; identical output

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