ADR 0187: tidy checks diagram geometry in the Markdown it formats

Amendments.

• ADR 0321 — block semantics: the shared scanner now enrolls box-drawing content in fenced and indented code blocks. The freeform opt-out remains an explicit fenced info-string tag.

Status: accepted

The bundled skeleton asks each project's setup agent to draw the system map as an ASCII diagram, and agents keep editing fenced diagrams as prose: borders drift, columns shift, junctions detach. No formatter can repair this — formatters skip fence bodies, and the embedded Markdown formatter keeps them byte-for-byte — so diagram geometry had no owner in any project. This repository cured its own instance with a repo-wide sweep (tests/diagram_geometry_test.ts), but a guard in this repository's test suite ships to nobody.

discern tidy scans every Markdown target it formats with the shared scanner (src/lib/diagram_geometry.ts): inside any fenced block containing a corner or junction glyph, vertical connections must be reciprocated (label text anchors a line; space does not) and arrowheads must sit on their shafts. Findings fail the result under diagrams_misaligned, one diagnostic per glyph with file, line, and column. Formatting writes still apply, so the tree converges while the run fails. A fence whose info string carries freeform is exempt.

tidy does not redraw diagrams. Two misaligned borders admit more than one faithful repair, so an auto-fix would guess at intent; the check names the glyph and leaves the fix with the author.

• Every project with discern tidy in its format job gets the check on upgrade, with no configuration. A map whose diagrams were already broken fails its next gate run at the fix stage with per-glyph pointers — the intended surfacing, and cheap while discern has no installed base.
• The repo-wide sweep and tidy import one scanner module, so this repository's gate and the shipped check cannot disagree about what "aligned" means.
• Deliberate character art needs the freeform tag; the failure message names it.
• The scanner's tolerances (label anchoring, corner-triggered fences, arrowhead point-facing) are the compatibility contract with real diagram idioms; tightening them is a behavior change for every project and deserves its own record.