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/tests/host.spec.ts

@@ -156,10 +156,10 @@ describe('readHostSource', () => {
     await expect(readSource('.')).rejects.toThrow(/not a regular file/)
   })
 
-  it('rejects an oversized source', async () => {
+  it('rejects an oversized source and reports the observed lower bound', async () => {
     await writeFile(join(ws, 'big.ts'), 'x'.repeat(100))
     await expect(readSource('big.ts', 10)).rejects.toMatchObject({
-      message: 'source "big.ts" exceeds the 10-byte limit',
+      message: 'source "big.ts" exceeds the 10-byte limit; reading stopped after 100 bytes',
     })
   })