Implement SAND-WP-0006: SaaS payments, routing, and ext.saas-stub

Add credits store, metering on create/destroy, extension routing resolver,
metered SaaS stub extension, burst/saas profiles, credits CLI, docs, and tests.

src/sandboxer/models.py

@@ -86,10 +86,32 @@ class ProfileMetadata(BaseModel):
     observability: Literal["none", "canary"] = "none"
 
 
+class RouteSpec(BaseModel):
+    strategy: RouteStrategy = RouteStrategy.EXPLICIT
+    extensions: list[str] = Field(default_factory=list)
+    max_cost_per_hour_usd: float | None = None
+
+
+class MeterQuote(BaseModel):
+    extension_id: str
+    estimated_usd: float
+    unit: Literal["per_hour", "per_session"] = "per_hour"
+    duration_s: int = 3600
+
+
+class MeterRecord(BaseModel):
+    pricing_model: Literal["self-hosted", "metered"] = "self-hosted"
+    estimate_usd: float | None = None
+    actual_usd: float | None = None
+    duration_s: float | None = None
+    currency: str = "USD"
+
+
 class Profile(BaseModel):
     id: str
     version: str
     extension: str
+    route: RouteSpec | None = None
     isolation: IsolationSpec = Field(default_factory=IsolationSpec)
     network: NetworkSpec = Field(default_factory=NetworkSpec)
     workspace: WorkspaceSpec = Field(default_factory=WorkspaceSpec)
@@ -130,6 +152,7 @@ class Reachability(BaseModel):
     remote_dir: str | None = None
     compose_project: str | None = None
     host: str | None = None
+    endpoint: str | None = None
 
 
 class SandboxStatus(BaseModel):
@@ -142,6 +165,7 @@ class SandboxStatus(BaseModel):
     reachability: Reachability | None = None
     inputs: dict[str, str] = Field(default_factory=dict)
     error: str | None = None
+    meter: MeterRecord | None = None
     telemetry: dict | None = None  # IntrospectionReport JSON when canary
     created_at: datetime
     updated_at: datetime