Run each application instance on its own dedicated thread (ADR 0009)

[pragmatrix]

Why

Every instance future ran on the shell's shared multi-thread runtime. Each instance iteration is one long synchronous poll body — select, shape lines, update the view, submit — and tokio can't preempt synchronous work, so in debug builds an instance under an output burst starved the desktop completely. mt carried an unconditional task::yield_now() per iteration as a mitigation, which is overhead when the desktop is idle and not a real fix.

Summary

• Each instance now runs on its own named OS thread (massive <id>) with its own tokio runtime, built inside the thread closure so the runtime's lifetime encloses block_on (dropping it early would abort spawn_blocking pty readers).
• Application gained RuntimeKind (CurrentThread default, MultiThread opt-in for instances with internal parallelism) via Application::with_runtime_kind; the desktop exports it.
• The instance TaskContext — including the shaping context — is created on the desktop task and moved into the thread explicitly, since the instance thread runs outside the desktop's task-locals.
• Instance threads are detached: completions arrive on an mpsc channel that join_next() drains; shutdown stays cooperative and the process end reaps unresponsive threads.
• The thread body is one fallible function driven to completion before a single completion report, so any setup failure (e.g. a failed runtime build) still reaches join_next instead of hanging it.
• Panic payloads from instance threads are converted to messages with the panic-message crate (zero dependencies, mirrors rustc's own unpacking logic) instead of a hand-rolled downcast chain.
• ADR 0009 records the decision and rejected options; the old plan doc is folded into it.

Validation

• cargo check -p massive-desktop, cargo fmt --check -p massive-desktop clean
• cargo test -p massive-desktop: 46 passed
• Runtime flavors verified empirically: nested current_thread runtime runs block_on with timers and cross-thread wakes (ADR 0009, "Considered options")

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