feat: add tmc::mux_many and tmc::mux_tuple

[tzcnt]

These replace and enhance the old result_each() overloads (this is a breaking change):

• tmc::spawn_many().result_each() -> tmc::mux_many
• tmc::spawn_tuple().result_each() -> tmc::mux_tuple

What's the same:

• mux_* is an lvalue awaitable that allows you to co_await it multiple times. Each time it returns the index of a single ready awaitable, which you can then access with operator[] / get().
• The group size is limited to 63 (or 31, on 32-bit) due to use of an atomic synchronization bitset.
• You must ensure that all active awaitables are finished (co_await mux == mux.end()) before the mux is destroyed.

The crucial new feature:

• You can .fork() a new awaitable to replace one that has completed, while other awaitables in the group are still running.
• Unlike tmc::select(), active (not finished) awaitables don't get canceled when a different awaitable completes or is replaced.
• Replacing awaitables is completely optional - you can await a mux that is not fully active.
• You can use this to:
  • maintain a constant level of concurrency
  • join streams from multiple queues
  • maintain a wait on a background signal while doing work
  • implement a timeout-based batch processor (see example batch_processor.cpp)

It also offers:

• You can customize individual task execution with fork(awaitable, executor, priority).
• You can fork_clang() to re-initiate awaitables with HALO, and it works in a loop as long as each is within a switch statement associated with the specific index that it's replacing.
• is_active() / active_bitset() methods let you track which awaitable slots are live.

What's lost from the old result_each():

• You cannot customize an entire mux group initial dispatch with mux_many().run_on(ex).with_priority(2). If you want to customize the tasks you must construct it empty and then fork(task, ex, 2) each awaitable.

Documentation:

• https://www.fleetcode.com/oss/tmc/docs/dev/awaitables/mux_many.html
• https://www.fleetcode.com/oss/tmc/docs/dev/awaitables/mux_tuple.html

I've also added an AGENTS.md / CLAUDE.md to this repo, for library users. It's fairly lightweight and is just designed to give your agent a headstart when using the library.