fix: preserve MIDI Manager dispatch frames

[benletchford]

Summary

• distinguish the legacy MIDI Manager $0004 selector family from Sound Manager routines
• implement the MIDI 2.0 version, sign-in, port, connection, wake-up, and stop-time selectors as a generic silent facade
• preserve each selector's documented Pascal stack frame and return values
• add regression coverage for the version result, return slots, reference number, and stack movement

Root cause

SoundDispatch previously routed only on the selector's routine byte. That made $00000004 (MIDIVersion) fall through to SndStartFilePlay, while later MIDI selectors retained their arguments because the MIDI family does not encode parameter sizes in the selector. A Director application eventually returned through those arguments to an invalid PC.

The new handling uses the Universal Interfaces 3.4 MIDI.h declarations for the legacy ABI. Systemless exposes a silent MIDI facade: clients can register and connect ports without corrupting execution, while packet transport and timer callbacks remain inactive.

Validation

• cargo test --lib — 2,951 passed, 3 ignored
• cargo check --no-default-features
• cargo fmt --check
• replayed the affected Director title through 2,102 guest ticks to its stable interactive gameplay interface

Closes #583

[benletchford]

Superseded by #586, which contains the evidence-backed fix for #583 and is ready for review.