fix(pty,subprocess,lsp): address review round on cancellation, lookup, and contracts

- pty-local: a pre-write inspection rejection no longer releases a canceled
  send while its foreground SIGINT is in flight; the interrupt path retains
  the slot and its post-signal tail resumes polling. Regression pins the
  failure-shaped cancellation and a close-during-write release.
- pty-local: SEND_ACTIVE names which provider operation is draining; README
  states the never-settling-provider outcome (slot retained, close recovers).
- subprocess-local: resolveExecutable rejects relative paths containing
  separators instead of expanding them per PATH entry with a misleading
  not-found error; seam JSDoc pins the rule for every provider.
- subprocess-local: LocalTerminalHandle documents why the seam's
  in-flight-join promise holds without operation tracking.
- lsp-local: the oversized-source diagnostic reports the observed byte lower
  bound; README documents that processId: null trades away server-side
  orphan cleanup after a hard-killed harness.
- pty/subprocess seams: cross-reference the twin five-member signal unions.
- pty-local: TODO markers for send-state consolidation and initialize-race
  relocation.

packages/subprocess/subprocess-local/src/index.ts

@@ -77,6 +77,11 @@ export class LocalSubprocessService extends SubprocessService {
     signal?.throwIfAborted()
     const environment = childEnv(env)
     const absolute = isAbsolute(command)
+    if (!absolute && (command.includes('/') || (process.platform === 'win32' && command.includes('\\')))) {
+      throw new Error(
+        `subprocess-local: command ${JSON.stringify(command)} is a relative path; use an absolute path or a bare PATH name`,
+      )
+    }
     const candidates = absolute ? [command] : this.executableCandidates(command, environment)
     for (const candidate of candidates) {
       signal?.throwIfAborted()

packages/subprocess/subprocess-local/src/terminal.ts

@@ -24,7 +24,14 @@ function signalName(number: number | undefined): NodeJS.Signals | null {
   return null
 }
 
-/** A local terminal whose process-session ownership stays below the PTY backend. */
+/**
+ * A local terminal whose process-session ownership stays below the PTY backend.
+ * The seam's terminate() promise — no write, inspection, or signal in flight
+ * after settlement — holds here without operation tracking only because every
+ * handle call completes synchronously under the hood (node-pty write, ps-based
+ * inspection). A first genuinely asynchronous step in any handle call must add
+ * the tracking a remote provider needs.
+ */
 export class LocalTerminalHandle implements SubprocessTerminalHandle {
   readonly pid: number
   readonly output = new PassThrough()