(fix): Scope-aware runtime escape detection

[mgyoo86]

Problem

Runtime escape detection (RUNTIME_CHECK=1) iterated all pool vectors when
checking for escaping arrays. This caused false-positive PoolRuntimeEscapeError
for arrays acquired in an outer @with_pool scope and used inside a nested
inner scope — a valid and common pattern.

@with_pool pool begin
    v = acquire!(pool, Float64, 100)      # outer scope
    @with_pool pool begin
        w = acquire!(pool, Float64, 50)   # inner scope
        result = compute!(v, w)           # v is safe here, but was flagged ✗
    end
end

Additionally, the direct-rewind @with_pool macro validated before cleaning
up leaked inner scopes, causing _scope_boundary to use an incorrect (leaked)
depth when an inner scope threw without rewind.

Solution

1. _scope_boundary — scope-aware overlap check

New @inline function that uses the existing checkpoint stack to compute the
boundary index: vectors at 1:boundary belong to outer scopes and are skipped.

@inline function _scope_boundary(tp::AbstractTypedPool, depth::Int)
    @inbounds if tp._checkpoint_depths[end] == depth
        return tp._checkpoint_n_active[end]
    end
    return tp.n_active   # no checkpoint → nothing acquired here → all safe
end

Applied to all three backends:

• CPU: _check_pointer_overlap, _check_bitchunks_overlap
• CUDA: _check_tp_cuda_overlap
• Metal: _check_tp_metal_overlap

2. Macro reorder — leaked scope cleanup before validation

In the direct-rewind @with_pool expansion (both block-form and function-form),
leaked inner scope cleanup now runs before _validate_pool_return:

Before: execute → validate → cleanup leaked → rewind
After:  execute → cleanup leaked → validate → rewind

This ensures _scope_boundary always sees the correct _current_depth.

3. Robust poison for custom isbits types

_poison_fill! now handles custom isbits structs via duck-type zero
(0 * first(v)) when zero(T) is not defined, and skips poisoning entirely
for non-isbits reference types (where resize!(v, 0) handles invalidation).

[Copilot]

Pull request overview

This PR fixes false-positive runtime escape detection in nested @with_pool scopes by making overlap checks scope-aware (only checking vectors acquired in the current scope), reorders direct-rewind macro cleanup to normalize leaked inner scopes before validation, and improves poisoning behavior for custom element types.

Changes:

• Add _scope_boundary and apply it to CPU/CUDA/Metal overlap checks so outer-scope arrays aren’t flagged in inner scopes.
• Reorder direct-rewind @with_pool expansion to clean leaked inner scopes before calling _validate_pool_return.
• Extend _poison_fill! to handle some custom isbits structs and skip poisoning for non-isbits element types; add comprehensive CPU/CUDA/Metal tests for scope-depth validation.

Reviewed changes

Copilot reviewed 11 out of 11 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
src/debug.jl	Adds _scope_boundary, updates pointer-overlap to be scope-aware, and changes _poison_fill! fallback behavior.
src/macros.jl	Reorders leaked-scope cleanup to occur before runtime validation in direct-rewind macro paths.
src/bitarray.jl	Makes BitArray chunk overlap checks scope-aware (Julia ≥ 1.11 path).
ext/AdaptiveArrayPoolsCUDAExt/debug.jl	Adds scope-aware boundary to CUDA overlap checks by threading current_depth.
ext/AdaptiveArrayPoolsMetalExt/debug.jl	Adds scope-aware boundary to Metal overlap checks by threading current_depth.
test/test_scope_depth_validation.jl	New CPU test suite covering nested scopes, containers, views/reshape parent chains, leaked scopes, and poison behavior.
test/runtests.jl	Includes the new CPU scope-depth validation test file.
test/cuda/test_scope_depth_validation.jl	New CUDA test suite mirroring key CPU scope-depth validation scenarios.
test/cuda/runtests.jl	Includes the new CUDA scope-depth validation test file.
test/metal/test_scope_depth_validation.jl	New Metal test suite mirroring key CPU scope-depth validation scenarios.
test/metal/runtests.jl	Includes the new Metal scope-depth validation test file.

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[codecov]

Codecov Report

❌ Patch coverage is 96.55172% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 96.61%. Comparing base (d73eac7) to head (3e95f31).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/macros.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #36      +/-   ##
==========================================
+ Coverage   96.55%   96.61%   +0.06%     
==========================================
  Files          14       14              
  Lines        2726     2748      +22     
==========================================
+ Hits         2632     2655      +23     
+ Misses         94       93       -1     

Files with missing lines	Coverage Δ
src/bitarray.jl	91.11% <100.00%> (+0.30%)	⬆️
src/debug.jl	95.70% <100.00%> (+0.77%)	⬆️
src/legacy/bitarray.jl	94.59% <100.00%> (+0.15%)	⬆️
src/macros.jl	97.98% <50.00%> (ø)

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