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/.env.example

@@ -78,3 +78,18 @@ AGENT_VERIFY_MAX_CHECKS=20
 AGENT_VERIFY_SEVERITIES=critical,high
 AGENT_VERIFY_REASONING_EFFORT=low
 VERIFY_MAX_TOKENS=8192
+
+# Text-layer grounding (deterministic PDF text layer via PyMuPDF)
+# TEXT_LAYER_ENABLED: master switch for text-layer extraction/grounding
+# TEXT_LAYER_MIN_CHARS: below this per page the sheet stays vision-only
+# TEXT_LAYER_MAX_CHARS: cap of text layer injected into the extractor prompt
+# VERIFY_TEXT_MAX_CHARS: cap of the text-layer excerpt in verify scopes
+# VERIFY_HI_DPI_CROPS: evidence-located high-DPI crops in the verifier
+# VERIFY_CROP_DPI / VERIFY_CROP_MARGIN_PTS: crop render DPI / padding (PDF points)
+TEXT_LAYER_ENABLED=true
+TEXT_LAYER_MIN_CHARS=20
+TEXT_LAYER_MAX_CHARS=12000
+VERIFY_TEXT_MAX_CHARS=8000
+VERIFY_HI_DPI_CROPS=true
+VERIFY_CROP_DPI=300
+VERIFY_CROP_MARGIN_PTS=36