fix(lsp): address codex review round 1

Lifecycle and safety fixes from the external review:
- Observe abort while awaiting the initialize handshake, so a server that never
  replies can't defeat the tool-timeout signal.
- On an aborted request the server won't cancel, tear the instance down after a
  bounded grace instead of releasing the serialized queue with work still live
  (prevents overlapping document lifecycles).
- Re-check provider disposal after the canonicalize/read awaits so a query can't
  spawn an unowned server after disposeAll().
- Read the source through one open handle (stat + read on the same fd) to close
  the realpath-vs-read TOCTOU; decode with a fatal UTF-8 decoder so a legitimate
  U+FFFD is not misclassified as invalid.
- Validate and read the source BEFORE spawning a server (pre-start rejection).
- Require an explicit openClose for option-form textDocumentSync.
- Reject nonpositive teardown budgets and non-executable absolute commands at
  load; surface unsupported operations as structured LSP_UNSUPPORTED_OPERATION.
- Retain the stderr tail (fatal diagnostics land at exit), not the prefix.
- Catalog the seam vocabulary in docs/core-data-structures/lsp.md.

packages/lsp/lsp-local/src/connection.ts

@@ -174,8 +174,9 @@ export class LspConnection {
   }
 
   private onStderr(chunk: Buffer): void {
-    if (this.stderr.length >= this.spec.maxStderrBytes) return
-    this.stderr = (this.stderr + chunk.toString('utf8')).slice(0, this.spec.maxStderrBytes)
+    // Retain the TAIL, not the prefix: a language server's fatal diagnostic usually appears just
+    // before it exits, so the final bounded segment is the useful one.
+    this.stderr = (this.stderr + chunk.toString('utf8')).slice(-this.spec.maxStderrBytes)
   }
 
   private dispatch(message: unknown): void {