Only adjust the drag hoverCount on an actual hover transition

[raineorshine]

Second finding from the audit prompted by #4969 (a cleanup that latched state because React re-runs cleanups on every dependency change, not only on unmount). Independent of #4975.

Problem

useDragLeave shares a module-level hoverCount across every drop target and debounces a clear of state.hoveringPath when it reaches zero. Two defects:

1. Every effect run was treated as a hover transition. Any run that was not a false → true change of isDeepHovering fell into the else branch and decremented the shared count:

if (isDeepHovering && !prevIsDeepHoveringRef.current) {
  hoverCount += 1
} else {
  hoverCount = Math.max(hoverCount - 1, 0)
  if (hoverCount === 0) debouncedSetHoveringPath?.()
}

The effect runs on mount, so any thought mounting mid-drag decrements the count. With one target genuinely hovered, the count goes to zero and hoveringPath is cleared — the blue drop indicator disappears while the cursor is still over the target.

2. The cleanup claimed to be an unmount handler but wasn't. It was commented // Cleanup on unmount while the effect listed four dependencies, so React ran it before every re-run. It also never decremented, so a drop target unmounted mid-drag leaked its count and hoveringPath was then never cleared.

Both are reproduced by the new tests, which fail on main:

× keeps hoveringPath while a drop target is hovered and an unrelated thought mounts
× clears hoveringPath when a hovered drop target unmounts

Solution

• Guard the count on isDeepHovering !== isCountedRef.current, so only a real enter or leave adjusts it. Mounts and canDropThought changes no longer touch the shared count.
• Move the release into its own effect with empty deps, where it decrements this target's contribution. The empty deps are what make it an unmount handler; that is why it can't be folded back into the main effect.

Verification

• New hook tests: 4 pass with the fix; the two above fail without it. The other two are guards that the count still reaches zero on leave (they pass either way, so they are guarding, not proving).
• yarn lint:tsc, eslint on the changed files
• Full unit suite: 1748 passed, 0 failed.
• docs/drag-and-drop.md updated with the counting rule.

Not verified: I could not run the Puppeteer drag-and-drop suite locally, so the change is verified against the hook's semantics and unit tests rather than a real drag. Worth a manual drag before merging, particularly nested drop zones (Thought and useDragAndDropSubThought each register a useDragLeave).

🤖 Generated with Claude Code