Remove an unneeded bounds check (that logically cannot ever be violated) from the base algorithm

[ExplodingCabbage]

There was never a need for the 0 <= addPathNewPos here:

        if (addPath) {
          // what newPos will be after we do an insertion:
          const addPathNewPos = addPath.oldPos - k;
          canAdd = addPath && 0 <= addPathNewPos && addPathNewPos < newLen;
        }

It will always be greater than 0. We start not having consumed even character 0 of the new string (newPos = -1), but the moment we add a single character, we are at newPos = 0 or more and there's no way to go back.

(And empirically, just removing this check doesn't break any tests.)

[Copilot]

🟢 Approval recommended

The maintained path invariant guarantees the removed lower bound cannot be violated.

0 open findings

What changed in this PR

Removes a redundant bounds check from the core Myers diff algorithm.

Changes:

• Simplifies insertion-path validation while preserving the upper-bound check.

File	Description
src/​diff/​base.ts	Removes the logically unnecessary lower-bound calculation.

🧠 Review effort: Balanced

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