feat: add runtime-agnostic select macro

[tisonkun]

Summary

Add an opt-in asyncband::select! macro for waiting on heterogeneous IntoFuture branches without choosing an executor. This makes it possible to compose Asyncband channels and synchronization with caller-provided stop notifications and deadlines.

The draft includes branch conditions, an all-disabled else fallback, random starting order, explicit biased; priority, and a runnable select_messages example:

cargo run --package examples --example select_messages

Design Notes

• Bindings are irrefutable: receive a Result and match it in the handler. A completed error or nonmatching payload is never silently discarded to continue selecting.
• Branches share the parent task's waker and are pinned separately with safe Rust. Owned futures are dropped before entering the selected handler; passing a pinned borrow preserves an unfinished operation across selections. Dropping does not roll back prior side effects.
• Default polling scans circularly from a random starting branch using the feature-local fastrand dependency. This reduces fixed-order bias without promising uniform selection or starvation freedom. biased; scans in source order.
• Conditions are evaluated once before branch construction. Disabled expressions are still evaluated and converted, but never polled. else means all branches are disabled, not merely pending.
• The declarative macro supports up to 32 branches without a proc-macro or custom unsafe pin projection. Timers remain caller-owned; dynamic selection sets and a nonblocking default branch are outside this API.

Validation: workspace tests on nightly and Rust 1.86.0; cargo x lint; cargo x check; cargo x build --locked; the runnable example; and an external Rust 1.86.0 consumer using a renamed dependency and 32 branches. Fifteen integration tests cover cancellation, resource cleanup before handlers, retained futures, waker replacement, wakeups during polling, branch conditions, and enclosing control flow.