fix: close figure diagnostics and render contract gaps

[moonweave]

What changed

• Add bounded line-segment/text crossing evidence for thin-line geometry cases while preserving the legacy artist_overlaps and informational legend_data_collision contracts.
• Add Nature-family fixed-canvas and explicit bbox_policy behavior, plus narrow-log minor-label suppression for journal targets.
• Make runtime snapshots and durable project results explicit in project-render responses, with safe URI/path sanitization and schema coverage.
• Clarify the public publication-readiness-v1 to internal v2 projection and fail-closed not_applicable geometry semantics.
• Split new policy/contract helpers to keep the architecture inventory below its module-size signal; refresh generated MCP references and witnesses.

Why

Issues #230, #231, and #232 exposed gaps where diagnostics could miss thin line/text crossings, style geometry contracts were ambiguous, or runtime artifacts could be confused with durable project results. The patch adds evidence and boundary clarity without forcing the LLM to make engine-side subjective decisions.

Validation

• 196 focused/integration tests passed, with 17 subtests.
• MCP rendering/read-only suites: 225 passed, 10 skipped, 11 subtests.
• Ruff, geometry rubric CLI, generated-reference check, architecture inventory, and git diff --check passed.
• Local uv/adjustText are unavailable; those environment-specific paths remain for GitHub Actions validation.

Release/tagging is intentionally not included; this PR should run the Windows and macOS CI gates first.