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/extractors.py

@@ -6,7 +6,7 @@ from typing import Dict
 from backend import config
 from backend.agents.base import AgentResult, AgentScope, AgentUsage, failure
 from backend.llm import call_json
-from backend.pipeline.extractor import _normalize_sheet
+from backend.pipeline.extractor import _normalize_sheet, _text_layer_block
 from backend.pipeline.sheet_index import _index_input
 from backend.prompts import (
     EXTRACTOR_SYSTEM_PROMPT,
@@ -65,7 +65,7 @@ class SheetExtractorAgent:
             page = scope.payload["page"]
             instruction = EXTRACTOR_USER_INSTRUCTION.replace(
                 "{sheet_hint}", str(scope.payload.get("sheet_hint") or "")
-            )
+            ) + _text_layer_block(page)
             parsed = _wrap_bare_list(self._call(instruction, page),
                                      page["page_number"])
             if not isinstance(parsed, dict):
@@ -79,7 +79,8 @@ class SheetExtractorAgent:
                 )
             if not isinstance(parsed, dict):
                 raise ValueError("no structured extraction returned")
-            sheet = _normalize_sheet(parsed, page["page_number"])
+            sheet = _normalize_sheet(parsed, page["page_number"],
+                                     page_text=page.get("text_layer"))
             return AgentResult(scope_id=scope.scope_id, artifacts=[sheet])
         except Exception as exc:
             return failure(scope, exc)