docrot v1.3.1 — tier chart + mixed-basis correction

v1.3.1 — tier chart + a public correction

New: the wave-3 report section now ends with "The same finding, drawn" — an inline SVG (no JS, no external requests) comparing the three measured tiers on like-for-like metrics: rot share per 100 scanned (26% / 27% / 30%) and third-party share of unique images (53% / 31% / 57%). Building the chart is what surfaced the bug below.

Correction (the datasets were right, the prose wasn't): earlier text compared shares across different bases — README said "77% vs 29%" (external-per-unique-image vs third-party-per-reference), the field log said "77% vs 50%", and the wave-2 report section even printed the self-contradictory "77% external vs 77%". Like for like, the true story is:

• external share of unique images: 77% → 77% → 75% (flat across tiers)
• third-party share of unique images: 53% → 31% → 57% (swings with where a corpus's outside images live)
• third-party share of references: 29% → 19% → 48%

All affected sentences in README, the live report, and the field log now carry the corrected numbers with a visible correction note. Every JSON dataset was and remains correct; no number in any finding or issue was affected — this was a prose-comparison error in the project's own narrative, caught by the project's own new chart.

Tests: 72 (+3 covering the tier chart: value order, labels, absence with a single tier).