feat: add alloc-tracking

[Ben Chatelain (phatblat)]

Resolves SDKS-1896
Extracted from getditto/ditto#18937

Summary

Adds an opt-in alloc-tracking Cargo feature that records allocation, deallocation, and Arc clone/drop events with backtraces to help diagnose use-after-free, double-free, and Arc refcount bugs in FFI code.

Changes

• New alloc-tracking feature (off by default) gated on a new optional backtrace dependency
• New src/alloc_tracking.rs module exposing AllocationTracker with:
  • track_alloc / track_free for Box/Vec/slice/char_p allocations (double-free and unknown-pointer-free panics)
  • track_arc_new / track_arc_clone / track_arc_drop for Arc refcount events
  • Global ALLOC_TRACKER behind a LazyLock<Mutex<…>>
• Instrumented the following types with feature-gated hooks on construction, clone, and drop paths:
  • ThinArc (src/arc.rs)
  • ThinBox (src/boxed.rs)
  • char_p::Box / CString conversions (src/char_p.rs)
  • slice_boxed (src/slice.rs), skipping zero-length and ZST allocations
  • Vec FFI wrapper (src/vec.rs)
  • Closure Arc/Box wrappers (src/closure/arc.rs, src/closure/boxed.rs)
• Added serial_test dev-dependency (tracker state is global, so tests must not run in parallel)

Notes

All instrumentation is behind #[cfg(feature = "alloc-tracking")] and has zero runtime cost when the feature is disabled.

cargo check --all-features does not work on this branch or on master — it activates the nightly feature (which requires a nightly toolchain) and hits a pre-existing serde visit_byte_buf mismatch in src/bytes.rs. Use the targeted commands below instead.

Test plan

• cargo check --features alloc-tracking
• cargo test --features alloc-tracking
• cargo check (default features, verify no regression from feature-gating)