feat(controls): B2 — grounded surface checks for absence-based CRA controls

Second slice of B (hybrid coverage): the controls violated by an *absence*
(no rate limiting, no security logging, no update-signature check) have no
syntactic pattern for semgrep, so we retrieve the code surface each governs and
let the grounded judge decide whether the control holds.

- controls/surface.rs: deterministic, bounded surface retrieval (keyword +
  window, capped per control) for cra-ai-6,11,24,27,28,29,30.
- grounded_surface_findings(): retrieve surfaces -> GroundedControlChecker ->
  net-new findings, each already tagged with its control and grounded to a real
  snippet (ground() drops anything not quoting verbatim code).
- orchestrator Stage 5d, gated on breakpilot.grounded_control_checks
  (BREAKPILOT_GROUNDED_CHECKS, default off) — absence detection is the least
  deterministic path, kept off until tuned against live scans.
- LUT: the 7 controls' notes now point to the gated grounded mechanism (kept
  needs_tooling; coverage stays honest until live-validated).

Local validation (real Qwen, temp 0), correct positive+negative discrimination:
  cra-ai-11 unprotected login  -> violates, CWE-307, grounded; protected -> false
  cra-ai-24 unlogged admin del -> violates, CWE-778, grounded; logged   -> false

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

compliance-core/src/config.rs

@@ -80,6 +80,11 @@ pub struct BreakpilotConfig {
     /// scale path and stays gated until verified live against a deployed
     /// master-controls catalog.
     pub semantic_mapping: bool,
+    /// Enable the **grounded surface** pass for absence-based controls (retrieve
+    /// the code surface a control governs, judge whether it holds). Off by
+    /// default: absence detection is the least deterministic path and stays gated
+    /// until tuned against live scans.
+    pub grounded_control_checks: bool,
 }
 
 impl Default for BreakpilotConfig {
@@ -89,6 +94,7 @@ impl Default for BreakpilotConfig {
             token: None,
             snapshot_dir: "/data/compliance-scanner/oscal".to_string(),
             semantic_mapping: false,
+            grounded_control_checks: false,
         }
     }
 }