fix(dismissable): Preserve target z-index when updating layer stack

[Hwacc]

This PR is aimed to address #3207

💣 Is this a breaking change (Yes/No): No

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Latest commit: e5bf8a2

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[kalisaNkevin]

Traced through why this happens: Dialog registers its positioner as a style target, not the primary layer node (layerStyleTargets: [getBackdropEl, getPositionerEl] in dialog.machine.ts), and remove() only clears the style-mirror on style targets — the primary content node's own --layer-index is untouched. So the positioner's z-index: var(--z-index) rule goes dead the instant --z-index is stripped, collapsing to auto mid-exit-animation. That matches the reported symptom exactly.

Freezing the resolved z-index as a literal inline z-index (which wins on specificity over the now-broken var() rule) is the right fix, and the second commit's removeProperty("z-index") in syncLayers correctly handles the case where the same node re-enters the stack later — without it a stale frozen z-index would override the real one on reopen. Three commits, but they read as one coherent fix.

Two things still missing, same gap as #3177:

• No changeset. Based on how fix(dismissable): restore layer style mirror after framework style rewrites #3177 documented the same package, something like:

  ---
  "@zag-js/dismissable": patch
  ---
  
  Fix `z-index` collapsing on layer style targets (e.g. a dialog's `positioner`)
  when a layer is removed, which hid stacked dialogs' exit animations behind
  lower layers.
  

• No regression test. If fix(dismissable): restore layer style mirror after framework style rewrites #3177 lands first, there'll be a test file in this package to add one to; right now there isn't.

One sequencing note: this and #3177 both touch the same two forEach blocks in remove()/syncLayers(). No logical conflict — one freezes z-index, the other manages a MutationObserver — but whichever merges second will need a rebase.

CI-wise: same story as #3177. Checked the Actions API — there's exactly one workflow run (Quality), status action_required. Needs a maintainer to approve it before there's any real signal; the unstable mergeable state reflects that pending approval, not an actual conflict.