feat: add gateway client with env-driven assembly

Includes config aggregation for multi-source env keys, from_env and
from_settings factories with explicit shared-backend injection,
gather_bounded, top-level exports, tightened import-linter layers with
the gate removed from the Makefile, and the finalized .env.example.

src/polygateway/backends/memory/breaker.py

@@ -51,8 +51,13 @@ class InMemoryGate:
         g.probe_owner = owner
         g.probe_expires = self._now() + self._cfg.probe_ttl_s
         return GateDecision(
-            source_name=source_name, allowed=True, state=GateState.HALF_OPEN,
-            epoch=g.epoch, is_probe=True, probe_owner=owner, retry_after_s=0.0,
+            source_name=source_name,
+            allowed=True,
+            state=GateState.HALF_OPEN,
+            epoch=g.epoch,
+            is_probe=True,
+            probe_owner=owner,
+            retry_after_s=0.0,
         )
 
     async def try_enter(self, source_name: str, owner: str) -> GateDecision:
@@ -63,23 +68,36 @@ class InMemoryGate:
         now = self._now()
         if g.state is GateState.CLOSED:
             return GateDecision(
-                source_name=source_name, allowed=True, state=GateState.CLOSED,
-                epoch=g.epoch, is_probe=False, probe_owner=None, retry_after_s=0.0,
+                source_name=source_name,
+                allowed=True,
+                state=GateState.CLOSED,
+                epoch=g.epoch,
+                is_probe=False,
+                probe_owner=None,
+                retry_after_s=0.0,
             )
         if g.state is GateState.OPEN:
             if now >= g.open_until:
                 return self._grant_probe(g, source_name, owner)
             return GateDecision(
-                source_name=source_name, allowed=False, state=GateState.OPEN,
-                epoch=g.epoch, is_probe=False, probe_owner=None,
+                source_name=source_name,
+                allowed=False,
+                state=GateState.OPEN,
+                epoch=g.epoch,
+                is_probe=False,
+                probe_owner=None,
                 retry_after_s=g.open_until - now,
             )
         # HALF_OPEN: 探针在途;租约过期则接管,否则拒绝(防惊群)
         if now >= g.probe_expires:
             return self._grant_probe(g, source_name, owner)
         return GateDecision(
-            source_name=source_name, allowed=False, state=GateState.HALF_OPEN,
-            epoch=g.epoch, is_probe=False, probe_owner=None,
+            source_name=source_name,
+            allowed=False,
+            state=GateState.HALF_OPEN,
+            epoch=g.epoch,
+            is_probe=False,
+            probe_owner=None,
             retry_after_s=g.probe_expires - now,
         )
 
@@ -96,8 +114,13 @@ class InMemoryGate:
 
     def _snapshot(self, g: _SourceGate, applied: bool) -> GateUpdate:
         return GateUpdate(
-            applied=applied, state=g.state, epoch=g.epoch, failure_count=g.fails,
-            retry_after_s=max(0.0, g.open_until - self._now()) if g.state is GateState.OPEN else 0.0,
+            applied=applied,
+            state=g.state,
+            epoch=g.epoch,
+            failure_count=g.fails,
+            retry_after_s=max(0.0, g.open_until - self._now())
+            if g.state is GateState.OPEN
+            else 0.0,
         )
 
     def _open(self, g: _SourceGate, reason: str) -> None:
@@ -118,7 +141,9 @@ class InMemoryGate:
         g.probe_expires = 0.0
         return self._snapshot(g, applied=True)
 
-    async def record_failure(self, entry: GateDecision, reason: str, force_open: bool) -> GateUpdate:
+    async def record_failure(
+        self, entry: GateDecision, reason: str, force_open: bool
+    ) -> GateUpdate:
         g = self._gate(entry.source_name)
         if not self._fenced(g, entry):
             return self._snapshot(g, applied=False)

src/polygateway/backends/memory/limiter.py

@@ -27,7 +27,9 @@ _WINDOW_S = 60.0
 class _MemoryPermit:
     """入场许可;release/settle 幂等(CHS _RedisPermit 同款 flag 语义)。"""
 
-    def __init__(self, limiter: InMemoryLimiter, source: str, lease_id: str, est: int, window: int) -> None:
+    def __init__(
+        self, limiter: InMemoryLimiter, source: str, lease_id: str, est: int, window: int
+    ) -> None:
         self._limiter = limiter
         self._source = source
         self._lease_id = lease_id