Commit 7831813 ("wpf-perf(big-win T4): pool AdornerLayer._zOrderMap
value snapshot") shipped a per-instance object[] snapshot buffer
shared between MeasureOverride and ArrangeOverride to eliminate ~170 MB
of per-pass DictionaryEntry[] allocations during MotionCatalyst
take-open.

Defect: Adorner.Measure / Adorner.Arrange callouts can re-enter the
same AdornerLayer's MeasureOverride/ArrangeOverride via a nested layout
pass. A naïve shared field lets the inner call's CopyTo overwrite the
outer pass's snapshot, and its terminal Array.Clear nulls the slots
the outer is still iterating — the outer then reads a null reference
and the layout throws, leaving MotionCatalyst with a completely blank
canvas on take-open.

Fix: lease pattern. Each call captures the current field value into a
local, immediately nulls the field (so any re-entrant call allocates
its own buffer rather than aliasing), iterates on the local, and at
end of pass restores its buffer to the field — keeping whichever
buffer (own or the one a nested call left behind) is larger.

Steady state on the non-re-entrant path remains zero-allocation: the
field holds the grown buffer, every subsequent call leases-clears-
copies-iterates-clears-restores in place. Re-entrant calls pay one
object[] allocation per nesting level, matching the worst case of the
pre-7831813a baseline.

Validated end-to-end via MCP UI screenshots on MotionCatalyst:
  - HEAD before fix: take-open shows fully black canvas
  - HEAD + this fix: identical to vanilla upstream/release/10.0
    (Carl Hansen golf swing, Frame 0/1240, both video viewports
     rendered, Pressure & Stance heatmap, Launch Monitor, all data
     boxes populated, playback toggles cleanly)

All 358 perf commits in PRs #1-#4 preserved.