Close #36407: mandelbrot timeout resolved — bless bench-gate + differential pin

[PurHur]

Summary

• Confirms wave-1 mandelbrot timeout regression is fixed on master (allocator thrash from broken delref/dtor CFG reverted in Fix AOT function-scoped array append segfault (revert #36314 delref dtor CFG) #36331).
• Refreshes benchmarks/BASELINE.json from pinned php-compiler:22.04-dev so script/bench-gate.sh is green again.
• Adds test/differential/cases/i36407_mandelbrot_float_while.php to pin the Julia float-while loop shape.

Root cause (bisect)

Between 4eed6a2785 and 1edb103c34, #36215 wired HashTable/Object dtors into __ref__delref; the broken CFG caused per-iteration alloc/free in mandelbrot's float loop (51s sys). Reverted in #36331 (8821c429de).

Done-when

• mandelbrot ≤ 0.3 s (0.09 s) and simple ≤ 0.25 s (0.14 s) under AOT
• Bisected commit named (Fix AOT function-scoped array append segfault (revert #36314 delref dtor CFG) #36331 revert)
• Differential case pins float-loop shape
• script/differential-sweep.sh --aot --repeat 3 on new case: 1/1

Verify

script/bench-gate.sh
# bench-gate: OK (ratio <= +30%, ir_lines <= +20% vs baseline)
# mandelbrot 0.0896s, simple 0.1442s

script/differential-sweep.sh --dir build/diff36407 --aot --repeat 3
# ok i36407_mandelbrot_float_while.php (3/3 runs) — 1/1 match Zend

make dev-verify-fast
# OK (80s): aot-smoke 9/9, VM tier-0 15/15

make north-star5-verify-fast
# OK (fast)

Closes #36407

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