[Copilot]

Thread teardown leaks the PAL-allocated alternate signal stack when application code replaces or disables its registration.

• Ownership: Save the allocation size alongside the PAL-owned pointer.
• Cleanup: Unmap that allocation regardless of the current registration, without unmapping a foreign stack.

Fixes #136853

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[Copilot]

🟡 Changes recommended

The cleanup change lacks Unix regression coverage with signal handling enabled to verify PAL mapping release and preservation of foreign mappings and registrations across unchanged, disabled, and replacement stack registrations.

1 open issue

• Add Unix regression tests for foreign-stack cleanup ownership · New

What changed in this PR

Fixes the CoreCLR PAL alternate signal stack leak described in #136853 when application code replaces or disables the stack registration before thread teardown.

Changes:

• Tracks the PAL-owned allocation size alongside its pointer.
• Unmaps the owned allocation independently of the current registration.
• Disables the registered stack only when it matches the PAL-owned stack, preserving foreign stacks.

File	Description
src/​coreclr/​pal/​src/​thread/​thread.cpp	Records allocation size and separates owned-memory cleanup from stack registration.
src/​coreclr/​pal/​src/​include/​pal/​thread.hpp	Adds and initializes the alternate stack allocation-size field.

_{Review effort: Balanced}

[janvorli]

LGTM, thank you!