feat(orchestrator): 会话状态机实现 + cancelled/resume（T3 架构审查整改）

- Issue3: 新建 src/genesis/state_machine.py（9 状态白名单转移）
  - cancel 记录 cancelled_from 进 cancelled 终态，resume 回中断点
  - 仅执行中状态可取消（awaiting_*/done 不可），非法转移抛 StateTransitionError
- 同步 agent-runtime-design.md §3.2（状态图/规则表 9 状态 + cancelled 行）
- 新增 10 用例（含覆盖补齐 2 项），全量 177 passed / 100.00%（981 stmts/252 br）

src/genesis/state_machine.py

@@ -0,0 +1,110 @@
+from __future__ import annotations
+
+from typing import Literal
+
+# 会话级状态集（9 个）：8 个设计态 + cancelled（T3 架构审查整改）
+SessionState = Literal[
+    "uploading",
+    "parsing",
+    "awaiting_parse_confirm",
+    "impact_running",
+    "awaiting_impact_confirm",
+    "writing",
+    "qa",
+    "done",
+    "cancelled",
+]
+
+STATES: frozenset[str] = frozenset({
+    "uploading",
+    "parsing",
+    "awaiting_parse_confirm",
+    "impact_running",
+    "awaiting_impact_confirm",
+    "writing",
+    "qa",
+    "done",
+    "cancelled",
+})
+
+# 合法转移白名单（不含 cancelled 的动态 resume 转移，见 SessionStateMachine.resume）
+_TRANSITIONS: dict[str, frozenset[str]] = {
+    "uploading": frozenset({"parsing"}),
+    "parsing": frozenset({"awaiting_parse_confirm", "cancelled"}),
+    "awaiting_parse_confirm": frozenset({"impact_running", "parsing", "cancelled"}),
+    "impact_running": frozenset({"awaiting_impact_confirm", "cancelled"}),
+    "awaiting_impact_confirm": frozenset({"writing", "impact_running", "awaiting_parse_confirm", "cancelled"}),
+    "writing": frozenset({"qa", "awaiting_impact_confirm", "cancelled"}),
+    "qa": frozenset({"done", "writing", "cancelled"}),
+    "done": frozenset(),
+    "cancelled": frozenset(),
+}
+
+# 可被取消的（非终态、非人工等待确认态之外全部执行中；done 不可取消）
+_CANCELLABLE: frozenset[str] = frozenset({
+    "parsing",
+    "impact_running",
+    "writing",
+    "qa",
+})
+
+
+class StateTransitionError(Exception):
+    """非法状态转移（对应 api-design §7 STATE_TRANSITION_INVALID 409）。"""
+
+
+class SessionStateMachine:
+    """会话级流程状态机：白名单转移 + cancelled/resume（T3）。
+
+    取消（cancel）从任意执行中状态进入 cancelled 终态，并记录中断前状态
+    （cancelled_from）；resume 从 cancelled 回到中断前状态，恢复后继续
+    正常白名单流转。人工等待确认态（awaiting_*）与 done 不可取消。
+    """
+
+    def __init__(self, initial: str = "uploading") -> None:
+        if initial not in STATES:
+            raise StateTransitionError(f"未知初始状态: {initial}")
+        self._state: str = initial
+        self._cancelled_from: str | None = None
+
+    @property
+    def state(self) -> str:
+        return self._state
+
+    @property
+    def cancelled_from(self) -> str | None:
+        """取消前的中断状态（resume 目标）；仅 cancelled 态非 None。"""
+        return self._cancelled_from
+
+    def transition(self, target: str) -> str:
+        """按白名单推进状态机；非法转移抛 StateTransitionError。"""
+        if target not in STATES:
+            raise StateTransitionError(f"未知目标状态: {target}")
+        allowed = _TRANSITIONS[self._state]
+        if target not in allowed:
+            raise StateTransitionError(
+                f"非法状态转移: {self._state} → {target}（白名单外）"
+            )
+        self._state = target
+        return self._state
+
+    def cancel(self) -> str:
+        """取消当前执行：进入 cancelled 终态，记录中断前状态。"""
+        if self._state not in _CANCELLABLE:
+            raise StateTransitionError(
+                f"当前状态不可取消: {self._state}（仅执行中状态可取消）"
+            )
+        self._cancelled_from = self._state
+        self._state = "cancelled"
+        return self._state
+
+    def resume(self) -> str:
+        """从 cancelled 恢复：回到中断前状态（cancelled_from）。"""
+        if self._state != "cancelled":
+            raise StateTransitionError(
+                f"仅 cancelled 状态可 resume（当前: {self._state}）"
+            )
+        assert self._cancelled_from is not None  # cancelled 态必有记录
+        self._state = self._cancelled_from
+        self._cancelled_from = None
+        return self._state