refactor(pool): simplify lifecycle and maintenance semantics

[tisonkun]

Summary

• Simplify bounded and unbounded pools around one private state implementation, remove approximate waiter accounting and duplicate mutex/retain helpers, and allocate bounded idle storage lazily.
• Make lifecycle semantics precise: last_used advances when a checkout is returned, retain invokes on_detached after releasing the pool lock, and status snapshots report exact tracked and idle object counts.
• Replace the spinning replenish path with fallible replenish_to, which reserves up to the immediately available capacity in one operation and reports manager creation errors.
• Add synchronous try_get for manually populated unbounded pools and support managed values that are Send without also requiring Sync.
• Add deterministic coverage for return timestamps, retention hooks and panic safety, queue order, cancellation policies, capacity under interleaved checkout/replenishment, lazy allocation, and trait bounds, plus focused Pool benchmarks.
• Reframe Asyncband in the README, crate docs, and package metadata as composable, runtime-agnostic concurrency building blocks. The documented boundary is reusable mechanism versus caller-owned execution and timing policy.

API notes

• bounded::PoolStatus::wait_count is removed because it was approximate and required bookkeeping on every checkout.
• bounded::Pool::replenish becomes replenish_to and returns Result<usize, M::Error>.
• unbounded::Pool<T>::try_get provides the non-waiting manual-pool path.

These APIs were introduced in the current unreleased cycle by #177, so this PR refines them before their first release.

Timeouts, task spawning, periodic retention/replenishment, and other runtime policy remain outside the pool and compose around its futures. This is a follow-up to #177 and is related to the broader scope discussion in #176.

Validation

• cargo x check
• cargo x test
• cargo x lint
• cargo bench -p benchmarks --bench benchmarks -- pool --test
• cargo package -p asyncband
• cargo tree -p asyncband --no-default-features --features pool -e normal (no normal dependencies)