Add render_confidence_heatmap() function that creates per-glyph
translucent colored cells representing extraction confidence.
Color coding:
- Red (#ef4444): confidence < 0.5 (low)
- Yellow (#eab308): 0.5 <= confidence < 0.8 (medium)
- Green (#22c55e): confidence >= 0.8 (high)
- Gray (#94a3b8): no confidence value (direct extraction)
Each cell includes data-* attributes (data-char, data-confidence,
data-span-index) for tooltip consumption by the frontend inspector
(Phase 7.9.6).
Implementation approximates per-glyph positions using span bbox
and character count, since the JSON schema only has span-level
confidence.
All unit tests pass. CSS class "heatmap-cell" enables frontend
toggling (Phase 7.9.3).
Closes: pdftract-67p2c
