feat: weak ref

[George-Miao]

No description provided.

[Copilot]

Pull request overview

Adds an optional weak feature to ThinCell (sync + unsync) to support non-owning Weak<T> handles, including separate strong/weak refcount tracking and accompanying tests/docs.

Changes:

• Introduces Weak<T> plus ThinCell::{downgrade,strong_count,weak_count} behind cfg(feature = "weak").
• Refactors refcount state representation to track strong + weak counts (and updates state tests).
• Adds sync weak-reference tests and updates CI/docs to build & test with all features.

Reviewed changes

Copilot reviewed 8 out of 8 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
src/lib.rs	Adds Weak<T> API and weak-aware drop/upgrade logic.
src/state/mod.rs	Adds strong/weak bit layout and weak-aware Snapshot helpers.
src/state/macros.rs	Implements strong/weak refcount ops (inc_weak, try_inc, dec_weak, etc.) and weak-specific tests.
src/sync/mod.rs	Adds Send/Sync impls for Weak<T> under the feature flag.
tests/sync.rs	Adds weak-gated multithreaded tests for downgrade/upgrade/count behavior.
README.md	Documents the new weak feature and provides an example.
Cargo.toml	Defines the weak feature and enables docs.rs all-features docs build.
.github/workflows/ci_test.yml	Runs cargo test --all-features in CI.

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