perf(Host验证): 为各类产物配置确定性预检

Host ValidationPolicy 为 Phase1、Phase2 和 Root Delivery 分别声明产物形状预检，并统一加入不可变引用与 SHA256 格式检查。硬性结构错误可在进入 Validator 模型前被拒绝。

script_build_host/src/script_build_host/agents/validation.py

@@ -91,6 +91,19 @@ class ScriptBuildValidationPolicy:
         return ValidationPlan(
             mode=ValidationMode.AGENT,
             validator_preset=preset,
+            preflight_rule_ids=(
+                "immutable-reference-present",
+                "sha256-digest-shape",
+                (
+                    "root-delivery-artifact-shape"
+                    if kind == ROOT_DELIVERY_KIND
+                    else (
+                        "phase-two-artifact-shape"
+                        if kind in PHASE_TWO_KINDS
+                        else "phase-one-artifact-shape"
+                    )
+                ),
+            ),
             max_evidence_queries=8,
             max_evidence_items_per_query=25,
             evidence_timeout_seconds=8.0,