Fix deep recursion stack overflow on Windows

[Koan-Bot]

Summary

Fixes t/10-deep_recursion.t crash on Windows (CPAN Testers: exit 255, 3/5 subtests failed).

Root cause: Deeply nested arrayrefs like [[[...]]] consume ~3 C stack frames per nesting level during recursive cloning. Windows has a 1MB default thread stack (vs 8MB on Linux/macOS), causing stack overflow well before the MAX_DEPTH safety limit was reached.

Three interrelated bugs fixed:

1. Platform-adaptive MAX_DEPTH: 4000 on Windows (safe for ~2000 nesting levels), 32000 elsewhere
2. MAX_DEPTH RV→AV routing: The depth check only handled SVt_PVAV but the [[[...]]] pattern alternates RV/AV — the limit was always first hit on an RV, falling through to SvREFCNT_inc(ref) which produced a shared (not deep-copied) inner structure
3. av_clone_iterative rewrite: Fixed two bugs — returned chain tail instead of root (lost nesting levels), and used stale seen-hash key

Test rewrite: Removed Devel::Peek and Devel::Size dependencies (unnecessary risk), tests 35000-level nesting built iteratively, added clone-independence mutation test.

CPAN Testers report: https://magpie.cpantesters.org/results/5483df40-720f-1014-847a-9de21b4fca93

Test plan

• All 205 tests pass locally (macOS, Perl 5.42.0)
• CI passes on Linux + Windows matrix
• Verify no memory leaks with deep structures (tested manually: 5 iterations of 20000-deep clone+free)

🤖 Generated with Claude Code

[Koan-Bot]

Recreated: Fix deep recursion stack overflow on Windows

The original branch koan.atoomic/fix-deep-recursion-windows had diverged too far from master for a clean rebase. The feature has been reimplemented from scratch on top of current master.

Branch koan.atoomic/fix-deep-recursion-windows has been force-pushed with the recreation.

Actions

• Read PR Fix deep recursion stack overflow on Windows #72: "Fix deep recursion stack overflow on Windows"
• Created fresh branch koan.atoomic/fix-deep-recursion-windows from origin/master
• Reimplemented feature from scratch
• Tests pass
• Force-pushed koan.atoomic/fix-deep-recursion-windows (recreated from scratch)

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