[tisonkun]

Summary

Several primitives retained verbatim or lightly adapted third-party documentation, examples, comments, and tests alongside Asyncband's own contracts, and internal::ValueCell still carried a Tokio-derived SetOnce wrapper.

• Replace asyncband/src/internal/value_cell.rs with a first-party ValueCell<T> built on AtomicBool publication and UnsafeCell<MaybeUninit<T>>, update LazyCell and OnceCell call sites, and remove value_cell.rs from third-party LICENSE notices.
• Clarify in LICENSE that the standard ASF header on asyncband/src/once/once_cell/mod.rs covers Asyncband's modifications while retained Tokio-derived portions remain under the MIT License, matching the existing Mutex, RwLock, and Semaphore entries.
• Standardize public API contract documentation (Returns, Errors, Cancellation, Panics, Deadlocks, and Examples) across all primitives and codify these documentation guidelines in AGENTS.md.
• Rewrite remaining upstream-derived doc comments, doctests, inline comments, and semaphore integration tests from Asyncband's observable contracts while preserving provenance headers where implementation code derives from upstream sources.