docs(authoring-contract): add viewport height budget and concrete via direction contract - #363

Problem and value

Current-main trigger: a candidate that passes validate --quality showcase with 0 errors / 0 warnings can still overflow the 1440×900 first screen in visual-check. The docs describe the goal qualitatively ("over-compressed Y layout", "vertical rhythm") but give no quantitative budget, so an author rediscovers the page-height arithmetic by trial and error after the spec is already frozen. Separately, the via direction contract is enforced by clean-flow/endpoint-side-direction but stated only abstractly ("a side is a direction contract"), so the exact constraint on via[0] coordinates is learnable only by triggering the error.

Intended outcome: an author can budget the viewport height at authoring time and write compliant via waypoints on the first try.

Evidence from a real authoring session on 2.16: official example web-app-rendered.html at viewBox 1080×588 (ratio 1.84) passes all four viewports, while my candidate at 1225×648 (ratio 1.89) overflows 1440×900 by 79px. A higher ratio overflowing while a lower one passes proves "keep the ratio above a threshold" is the wrong mental model; the determinant is absolute rendered height (viewer chrome ~51px + 930px reader-width × viewBoxH/viewBoxW + cards). Tightening card copy recovered 34px; stretching node x-coordinates (viewBox 1225→1315) recovered the remaining 11px.

Approach: docs-only — one new subsection and one bullet extension (below). Narrow documentation correction; per CONTRIBUTING "Choose the right path", no separate planning issue needed.

Stability impact

• Impact class: documentation only. No code, schema, defaults, or acceptance rules changed.
• Existing behavior preserved: all existing text unchanged except the one direction-contract bullet, which is strictly extended (original sentence retained verbatim).
• No unrelated changes: confirmed — single file, +23/−1.

Tests run

• Comparison base: main @ 1072200. Candidate: this branch (1 commit).
• Markdown structure verified: "Viewport height budget" nests under "Executable geometry rules" alongside "Spacing and labels" / "Repair order"; no heading regressions.
• No automated checks apply to prose; archify doctor unaffected (docs not loaded by the CLI).

Visual evidence

Not applicable — documentation-only change; no rendering path touched.

Generated artifacts

None; no generated files reference references/authoring-contract.md content.