fix(subprocess): tree-scoped escalation and byte-exact tails (Codex round 1)

(A1) terminate()/dispose()/service teardown keyed on direct-child settlement
could leak a TERM-trapping descendant that outlived the leader (Codex
reproduced it with a disowned trap-SIGTERM helper). kill()/terminate() now
gate on tree liveness instead of outcome settlement; the SIGKILL escalation
timer survives settle (unref'd, re-probing the tree); dispose's tier
quiescence is whole-tree exit via a bounded waitForExit; the service's live
set releases handles only when their tree is gone, and its teardown awaits
tree exit. Three new suites pin the survivor scenarios end to end
(terminate, dispose, service teardown).

(A2) the escalation branch is now real tested behavior — its ignore is gone;
the one remaining signalTree guard ignore states why it is unreachable
through the handle verbs.

(A3) docs contradictions fixed: the impl README's stale POSIX-only bullet
now states the contained best-effort Windows tree story; the lsp-local
README no longer claims taskkill failures stay visible (containment + the
tree-liveness wait is the actual contract); the architecture tables (en+zh)
list all three consumer families.

(B1) OutputCollector keeps a byte-exact tail across uneven chunk boundaries
(trim the head chunk instead of dropping it whole) — the LSP diagnostic-tail
contract; pinned by a cross-chunk test.

(B2) the subagent-acp coverage ignore is narrowed to exactly the
never-settling success arm.

docs/architecture.zh.md

@@ -28,7 +28,7 @@
 | `ctx.llm` | [`llm/`](../packages/llm/README.md) | 适配器注册表和模型流式调用 |
 | `ctx.tokenMeter` | [`llm/token-meter`](../packages/llm/token-meter/README.md) | 感知回放的单实例请求压力和会话表面压力 |
 | `ctx.bash` | [`bash/`](../packages/bash/README.md) | 前台和后台命令执行 |
-| `ctx.subprocess` | [`subprocess/`](../packages/subprocess/README.md) | bash 执行器之下受管理的子进程组 |
+| `ctx.subprocess` | [`subprocess/`](../packages/subprocess/README.md) | 供 bash 执行器、LSP host 与 ACP subagent 后端使用的受管子进程树 |
 | `ctx.pty` | [`pty/`](../packages/pty/README.md) | 按 owner 隔离的持久化终端会话 |
 | `ctx.sandbox` | [`sandbox/`](../packages/sandbox/README.md) | 同一执行环境内的进程限制（argv 包装、逐调用策略） |
 | `ctx.sandboxPolicy` | [`sandbox/`](../packages/sandbox/README.md) | 共享沙箱策略归属点 |