[RClaudiuM]

Fixes #1355

What

With enablePatches(), shrinking a draft array by assigning length and then growing it back to at least its base length produced no replace patch (and no inverse patch) for the truncated indices. So applyPatches(base, patches) kept the stale values, and the inverse patches did not restore the base. The same recipe with splice(0) was already correct.

Why

generateArrayPatches only emits a replace for an index below base_.length when assigned_ has it (or allIndicesReassigned_ is set). splice removes elements through deleteProperty, which marks each index. Assigning length goes through the set trap for "length" only, so the dropped indices were never marked.

Change

In the array set trap, when length shrinks, mark the dropped indices that exist in the base as assigned. Only indices below base_.length are marked, since those are the only ones the replace loop looks at. If the array stays short, nothing changes: the replace loop only runs up to the shorter length, and the dropped indices still get remove patches.

Tests

New cases in __tests__/patch.js (through runPatchTests, so they also check replay and reversal):

• length = 0 then d[1] = 2 (the case from the issue)
• length = 0 then length = 2
• length = 1 then push(4, 5) (control, already passing)
• length = 0 then the same values written back (control: no patches)

Before the fix, the first two fail (6 assertions). After, all pass.

Repro from the issue, run against the build:

before:
patches [{"op":"add","path":[1],"value":2}]
inverse [{"op":"remove","path":[1]}]
replay  [ 1, 2 ]
undo    [ <1 empty item> ]

after:
patches [{"op":"replace","path":[0],"value":undefined},{"op":"add","path":[1],"value":2}]
inverse [{"op":"replace","path":[0],"value":1},{"op":"remove","path":[1]}]
replay  [ undefined, 2 ]
undo    [ 1 ]

Checks run locally (Node 24): yarn test (src, build and flow), yarn coverage, yarn test:perf, prettier --check.