[alloc+atomic] Make ghost_tree_bstack_allocator thread-safe with BSTACK_FEATURE_ATOMIC

[williamwutq]

Description: This is the C mirror of #13

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
C Only: Yes
Fuzz: Yes
Safety Review: Needed: Crash Safety, Invariants, Thread Safety

[williamwutq]

LGTM

[Copilot]

Pull request overview

This PR updates the C implementation of ghost_tree_bstack_allocator_t to support safe cross-thread use when compiled with -DBSTACK_FEATURE_ATOMIC, matching the intent of the Rust-side change in PR #13. It introduces an in-memory mutex to serialize AVL-tree mutations and uses bstack_try_discard / bstack_try_extend_zeros for atomic tail operations, plus adds concurrency-focused test coverage and CI wiring.

Changes:

• Add an optional (feature-gated) in-memory mutex to ghost_tree_bstack_allocator_t and use it to serialize AVL mutations under BSTACK_FEATURE_ATOMIC.
• Switch tail resize/free paths to bstack_try_extend_zeros / bstack_try_discard under BSTACK_FEATURE_ATOMIC.
• Add concurrent stress tests for alloc/dealloc/realloc/bulk paths and run them in CI via a new make test-ghost-tree-atomic target.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
c/test_ghost_tree.c	Adds concurrent allocator stress tests gated by BSTACK_FEATURE_ATOMIC.
c/Makefile	Adds test-ghost-tree-atomic target to build/run ghost-tree tests against the SET+ATOMIC alloc library.
c/bstack_alloc.h	Documents thread-safety behavior and adds a feature-gated lock field to the allocator struct; updates magic string comment.
c/bstack_alloc.c	Implements mutex init/destroy helpers and makes ghost-tree allocator operations thread-safe under BSTACK_FEATURE_ATOMIC (including atomic tail paths).
.github/workflows/cci.yml	Runs the new atomic ghost-tree test target in CI (non-Windows).

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