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/src/index.ts

@@ -107,7 +107,9 @@ export abstract class SubprocessService extends Service {
   /**
    * Resolve one configured executable in this provider's execution world.
    * Absolute paths are verified; bare names use the provider's scrubbed PATH
-   * plus explicit environment overrides.
+   * plus explicit environment overrides. Relative paths containing separators
+   * are rejected: no current consumer defines which directory they would
+   * resolve against, so providers fail loud instead of guessing.
    * @param command - absolute executable path or bare PATH name.
    * @param env - explicit environment entries used for lookup.
    * @param signal - aborts remote or local lookup.

packages/subprocess/subprocess/src/types.ts

@@ -193,7 +193,11 @@ export interface SubprocessHandle {
   waitForExit(signal?: AbortSignal): Promise<boolean>
 }
 
-/** Signals supported by the terminal-process primitive. */
+/**
+ * Signals supported by the terminal-process primitive. Kept member-identical
+ * to `PtySignal` in `@deepseek-ai/dsh-pty` without a cross-seam dependency;
+ * change both together.
+ */
 export type SubprocessTerminalSignal = 'SIGINT' | 'SIGTERM' | 'SIGKILL' | 'SIGTSTP' | 'SIGHUP'
 
 /** A fully specified terminal-process spawn. */