One scheduler tier for cancellation, push notifications, and background work

[Firehed]

What is missing

The server handles one message at a time and only ever replies. Three behaviours need more than that:

• $/cancelRequest: drop a completion request the client has already superseded.
• textDocument/publishDiagnostics: push errors and warnings as the user types (Design: Diagnostics architecture (errors, warnings, type checking) #266).
• Background work, such as indexing, that does not freeze the editor.

The one extension point

Built one at a time, these become three scheduling paths and three ways to write to the client. That is feature × mechanism, the M×N shape this project forbids.

There is one scheduler, in the transport and dispatch tier, and it is the only place that:

• runs a unit of work and can cancel it (every unit carries a cancellation token);
• decides when a server-initiated message is sent (debounce, ordering);
• writes to the client, through the existing ClientConnection and OutgoingMessage channel.

A feature adds a job kind or a notification value. It never adds a loop, a timer, a fork, or a write to the transport. A change that does is rejected in review.

Constraints (RFC 1 §6)

• The resolution interior stays synchronous and correct without the scheduler.
• pcntl and ext-parallel are feature-detected with a synchronous fallback; Fibers and FFI may be relied on.
• Background results re-enter symbol state through SymbolSink only.
• When the rule freeze lifts (build manifest, last step), a deny entry confines TransportInterface::write() to the Client tier so a second output path cannot appear. The tier and its rule land together.

Acceptance

• A superseded request is abandoned and answered with RequestCancelled.
• A debounced notification reaches the client through ClientConnection; no other class writes to the transport.
• A background job cannot starve an interactive request, and can be cancelled.

When

After the last step of docs/architecture/build-manifest.md lands, and only when the first push feature (#266) is scheduled.

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This issue body was written by AI and has not been reviewed line by line by a human.