refactor(subprocess): keep terminate() as the seam's only termination verb

Delete kill(signal?) from SubprocessHandle: consumers stop a process only
through terminate()'s tree-scoped SIGTERM→graceMs→SIGKILL escalation
(idempotent, also driven by the spec's abort signal, a no-op once the tree
is gone). The single-signal verb had exactly one consumer family —
lsp-local — and what it bought there was a private re-implementation of
the same escalation. The internal kill closure stays in spawn.ts as the
dispose ladder's tier primitive; terminate() now routes through it too.

lsp-local collapses onto the seam's escalation:
- LspConnection replaces its terminate()/kill() pair with one terminate()
  that delegates to handle.terminate(). Behavior change: the
  framing-failure path terminates instead of instant SIGKILL, so a
  misbehaving server now gets SIGTERM plus the killGraceMs window to
  flush before SIGKILL.
- ConnectionSpec.pipeDrainGraceMs becomes killGraceMs: one grace, the
  spawn spec's graceMs, drives both the escalation window and post-exit
  pipe draining (the provider already passed killGraceMs for it).
- LspInstance.forceTerminate() drops its hand-rolled bounded first wait
  (LSP_KILL_GRACE) and escalateProcessTree (deleted with its export and
  unit test): the seam's escalation already commits to SIGKILL after
  killGraceMs, so only the unbounded quiescence awaits stay load-bearing.

Tests: kill()-shaped spawn specs become terminate()-shaped or fold into
the terminate() suites (group-wide delivery; the settled no-op case was
already pinned by 'terminate() after the tree died'); tree-survivor
coverage is intact. A stderr-'inherit' disposition test completes the
stdout/stderr symmetry so the scoped subprocess+lsp coverage gate stands
alone instead of leaning on subagent-acp's cross-package runs.

Docs: SubprocessHandle type-equiv block, seam/impl/group READMEs, and the
consumer-migration Agent Note lose the kill(signal?) vocabulary (zh pairs
re-recorded); cordis api/services catalogs regenerated.

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

@@ -30,11 +30,11 @@ export interface ConnectionSpec {
   /** Largest stderr tail retained for diagnostics. */
   readonly maxStderrBytes: number
   /**
-   * Bound (ms) for draining pipes a surviving helper still holds after the
-   * server exits; the instance passes its kill grace so exit observation is
-   * never slower than the escalation it feeds.
+   * The subprocess spec's `graceMs`: the SIGTERM→SIGKILL window of
+   * {@link LspConnection.terminate}'s escalation, and the bound for draining
+   * pipes a surviving helper still holds after the server exits.
    */
-  readonly pipeDrainGraceMs: number
+  readonly killGraceMs: number
   /** Static answer to every `workspace/configuration` item. */
   readonly configuration: unknown
 }
@@ -98,7 +98,7 @@ export class LspConnection {
         stdout: 'pipe',
         stderr: { maxBytes: spec.maxStderrBytes },
       },
-      graceMs: spec.pipeDrainGraceMs,
+      graceMs: spec.killGraceMs,
       // spec.env mixes the scrubbed base with explicit config entries; a
       // configured DSH_* fact takes the managed channel the seam reserves.
       ...splitEnvChannels(spec.env),
@@ -210,14 +210,9 @@ export class LspConnection {
     return this.nextId
   }
 
-  /** Request termination of the server's process tree (SIGTERM, no escalation). */
+  /** Terminate the server's process tree (the seam's SIGTERM→grace→SIGKILL escalation; idempotent). */
   terminate(): void {
-    this.handle.kill('SIGTERM')
-  }
-
-  /** Force termination of the server's process tree. */
-  kill(): void {
-    this.handle.kill('SIGKILL')
+    this.handle.terminate()
   }
 
   /**
@@ -235,9 +230,10 @@ export class LspConnection {
       messages = this.decoder.push(chunk)
     } catch (error) {
       // A framing/JSON failure corrupts the stream position irrecoverably: fail the instance and
-      // SIGKILL the whole group so helper processes don't outlive the leader.
+      // terminate the whole group so helper processes don't outlive the leader (SIGTERM first, then
+      // the kill grace's SIGKILL — a misbehaving server still gets its bounded flush window).
       this.fail(asError(error))
-      this.handle.kill('SIGKILL')
+      this.handle.terminate()
       return
     }
     for (const message of messages) this.dispatch(message)