Support N MIDI inputs, each with its own queue

[jeudine]

Summary

Generalizes MIDI input from a single optional port to N independent inputs, each with its own message queue and consumer thread, and lets handle_input forward MIDI straight to the output.

• run() now takes midi_in: Vec<MidiInParam> instead of Option<MidiInParam> (empty vec = standalone/no input).
• Conductor::handle_input / Context::handle_input gain a 0-based input_id (matching the input's position in the Vec) so messages can be routed per source.
• At most one input is the clock/transport source: the first one marked slave. Extra slave inputs are treated as message-only, with a warning.
• One consumer thread per input, each draining its own queue.

Direct forwarding from handle_input

Conductor::handle_input still returns a Vec<Instruction>, and the pause filter now depends on the instruction:

• Instruction::MidiMessage is forwarded straight to the MIDI output, including while paused.
• Every other instruction goes through the MidiController (buffered / step-scheduled) and is dropped while paused.

This lets a conductor echo or transform incoming MIDI immediately and even while paused, without the controller's clock-quantized note buffering. An earlier revision of this branch expressed it as a two-channel InputResponse struct, which has since been removed in favour of the single Vec<Instruction>.

Correctness / cleanup

• Fixed a condvar lost-wakeup in the consumer threads and the slave loop (predicate loop + drain-into-local via mem::take so the real-time midir callback is never blocked while the conductor runs).
• Introduced a NotifyQueue struct to replace the (Arc<Mutex<..>>, Arc<Condvar>) tuple.
• Removed the dead pause field from Context.
• Added MidiMessage::is_transport; added slave-mode multi-input and pause-forwarding tests.
• DeteTrack::play_step and get_notes_start_at_step no longer panic on a zero-length track. DeteTrack derives Default, so len = 0 was reachable from safe code and from deserialization, and both methods computed step % self.len. They now log a warning and return no instruction.

Documentation

• Added docs/architecture.svg, a single figure showing what the user implements (Conductor, Tracks) and what the engine does with it: the direct-forward path, the sequencer path, and transport. Embedded in both the mseq and mseq_core READMEs.
• Fixed the root README, whose rustdoc-style [Conductor] links all rendered as literal text, since the file is not included by src/lib.rs. They now resolve to docs.rs.
• Moved the usage example to the top of the README, made it play a note, and had it call ctx.start(). The sequencer starts paused, which was the one gotcha that left a first conductor silent and was documented nowhere. Replaced the DeteTrack::default() snippet, which panicked, with a real DeteTrack::new call.
• Merged the README's Overview and Conductor Trait sections, which said the same thing twice.
• mseq_core's README now says it has no MIDI I/O or run loop of its own; mseq_tracks gained a track table, an example and its feature flags.

Crate metadata and no_std

• mseq_tracks is now genuinely no_std when the std feature is off. It gated the file loaders but had no #![no_std] and pulled csv, midly, toml and fs-err unconditionally, so --no-default-features removed most of the API without making the crate usable on a bare-metal target.
• Dropped midir from mseq_tracks, which never referenced it.
• Gave each crate its own description. All three shared "Library for developing MIDI Sequencers.", which made them indistinguishable on crates.io.

Breaking changes

• run() signature: Option<MidiInParam> -> Vec<MidiInParam>.
• Conductor::handle_input gains an input_id: usize parameter.
• mseq_tracks dependencies used only behind std are now optional, so --no-default-features builds a smaller crate than before.

Testing

• cargo fmt --all --check, cargo clippy --workspace (all-features + no-default-features, -D warnings)
• RUSTDOCFLAGS="-W missing_docs -W rustdoc::all" cargo doc --workspace --no-deps
• cargo test --workspace --all-features (unit, integration, doctests)
• cargo build -p mseq_tracks --no-default-features --target thumbv7em-none-eabihf, which could not succeed before this branch

🤖 Generated with Claude Code

[AmdyTran]

LGTM