IDE MVP: ship a Rust .own LSP with broken-code tolerance and cancellation #264

Parent: #250
Recommended tier: strong
Depends on: #255 and preferably #256
Informed by: #263

Goal

Build the first IDE vertical as a pure-Rust LSP for .own files:

document text
  -> own-syntax
  -> own-cfg
  -> own-analysis
  -> own-diagnostics
  -> LSP diagnostics

This is the smallest end-to-end IDE path and the proving ground for spans, cancellation, stale-result handling and broken-code tolerance before the hybrid C# host.

MVP protocol surface

Implement at minimum:

• initialize/shutdown/exit;
• textDocument/didOpen;
• textDocument/didChange;
• textDocument/didClose;
• publishDiagnostics;
• document version tracking;
• cancellation of superseded analysis;
• configurable/debounced analysis where justified by measurement.

No completion, rename, references or code actions in the MVP unless they are essentially free and separately reviewed.

Parser/broken-code contract

An IDE analyzes incomplete code on every keystroke. The LSP must:

• never panic on truncated or malformed input;
• publish syntax diagnostics for recoverable errors;
• run semantic analysis on the safely recoverable portion where the existing parser contract permits it;
• avoid publishing diagnostics from an older document version;
• clear diagnostics when a document closes or becomes clean;
• preserve stable byte-offset/span mapping through edits.

If current own-syntax cannot satisfy a required recovery case, record the exact parser gap rather than hiding it in LSP glue.

Scheduling/cancellation

• New document versions supersede older work.
• Cancelled/superseded computations must not publish.
• Analysis of one document must not indefinitely block another.
• No unbounded task/thread creation on rapid edits.
• Server shutdown must terminate cleanly.

Diagnostic mapping

Map the Rust diagnostic model to LSP:

• severity;
• range;
• code;
• message;
• source Owen;
• related information where supported by the Evidence contract.

Line/column conversion must use the agreed source indexing contract and be tested with Unicode.

Integration tests

Drive the server through real JSON-RPC/LSP messages and cover:

• open clean document;
• open leaking/invalid document;
• rapid edit sequence where only the final version publishes;
• edit from error to clean;
• syntax error recovery;
• truncated token/expression;
• Unicode before/inside diagnostic range;
• close clears diagnostics;
• cancellation/supersession;
• two documents analyzed independently;
• graceful shutdown.

Performance targets

Use the budgets established by #263. Record:

• cold first diagnostic;
• warm edit-to-diagnostic;
• no-op change behavior;
• peak memory for representative documents.

Do not introduce salsa or another incremental framework merely by reputation. Add it only if the measured invalidation model and complexity justify it.

Guardrails

• .own only; no C# Roslyn integration.
• No new diagnostic semantics.
• No public Owen CLI cutover.
• No completion/rename feature expansion.
• No stale diagnostics.
• No swallowed panics or automatic process restart presented as correctness.

Acceptance

• A standalone Rust LSP binary runs on supported development platforms.
• Broken/incomplete .own input does not crash the server.
• Superseded document versions never publish stale diagnostics.
• Diagnostic code/message/range mapping is correct, including Unicode controls.
• Integration tests exercise real LSP messages.
• Measured edit latency meets the approved MVP budget or the gap is explicitly documented before merge.
• Workspace DAG and CI remain green.