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/lsp/lsp-local/src/host.ts

@@ -107,7 +107,11 @@ export async function readHostSource(
     throwIfAborted(signal)
     throw new Error(`source "${filePath}" could not be read: ${messageOf(error)}`, { cause: error })
   }
-  if (bytes > maxDocumentBytes) throw new Error(`source "${filePath}" exceeds the ${maxDocumentBytes}-byte limit`)
+  if (bytes > maxDocumentBytes) {
+    throw new Error(
+      `source "${filePath}" exceeds the ${maxDocumentBytes}-byte limit; reading stopped after ${bytes} bytes`,
+    )
+  }
   throwIfAborted(signal)
   return {
     fileUrl: fs.fileUrl(target),