[alloc+atomic] Make GhostTreeBStackAllocator Sync under the atomic feature flag

[williamwutq]

Description: Under the flag atomic, making GhostTreeBStackAllocator (Rust) operations atomic and safe across threads with a mutex and bstack atomic methods such as try_extend_zeros and try_discard, implementing the rust Sync trait.

Important Feature: No
Type: Allocator - Optimization; Allocator - Concurrent
Magic Number: ALGT
Bulk: No
Tests: Included
Feature Flags: alloc + set
Breaking change: No
New Types: None
Rust Only: Yes
Fuzz: Yes
Safety Review: Needed: Crash Safety, Invariants, Thread Safety

[williamwutq]

LGTM

[Copilot]

Pull request overview

This PR updates GhostTreeBstackAllocator to support safe concurrent &self usage under the atomic feature flag, making the allocator Sync by serializing AVL free-tree mutations with an internal mutex and using BStack’s atomic tail operations (try_extend_zeros, try_discard) where possible.

Changes:

• Add a Mutex<()> field behind cfg(feature = "atomic") and remove the !Sync marker in that configuration.
• Wrap AVL free-tree remove/insert operations in the mutex and restructure alloc/alloc_bulk to release the lock before potentially expensive extend/zero calls.
• Use try_discard / try_extend_zeros as lock-free fast paths for tail shrink/grow in realloc, dealloc, and dealloc_bulk, plus update the thread-safety docs and doctest gating accordingly.

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