feat: text-layer grounding (extractor authority, guard rescue tier, verifier oracle + hi-DPI crops)

- backend/text_layer.py: PyMuPDF text-layer extraction, fuzzy evidence
  bbox matching, 300-DPI crop rendering, coverage-gap signal
- extractor (classic + agent): TEXT LAYER block appended at call sites;
  grounding guard gains text-layer rescue tier (grounding=text_layer stamp)
- verifier: {text_layer} oracle excerpt + evidence-located hi-DPI crops
  replacing full-page images (fallback preserved, I2 guard intact)
- coverage gaps: text-bearing pages with zero extraction -> failed-scope
  gap findings (agent) / log-only (classic)
- config knobs: TEXT_LAYER_ENABLED/MIN_CHARS/MAX_CHARS, VERIFY_TEXT_MAX_CHARS,
  VERIFY_HI_DPI_CROPS, VERIFY_CROP_DPI, VERIFY_CROP_MARGIN_PTS
- tests: 22 new (text_layer unit, grounding/render, runner-level flow)
Spec: docs/superpowers/specs/2026-08-12-text-layer-grounding-design.md

backend/agents/verifier.py

@@ -55,7 +55,11 @@ class EvidenceVerifierAgent:
     def run(self, scope: AgentScope) -> AgentResult:
         try:
             finding = scope.payload["finding"]
-            instruction = render(VERIFY_USER_INSTRUCTION, {"finding": dumps(finding)})
+            instruction = render(VERIFY_USER_INSTRUCTION, {
+                "finding": dumps(finding),
+                "text_layer": scope.payload.get("text_layer_excerpt")
+                              or "(no text layer available for the cited sheets)",
+            })
             parsed = call_json(
                 system_prompt=VERIFY_SYSTEM_PROMPT,
                 user_text=instruction,