feat(verification): adopt Python renderer mechanics

[BjRo]

Why

Closes #177. The verification assessment renderer was the remaining shell-based deterministic mechanic in the plugin, limiting the shared Python quality gates and native Windows coverage established by #172.

What changed

Replaced the shell renderer with a dependency-free, UV-locked Python package while preserving its accepted inputs, byte-stable assessment body, exact report-link format, validation diagnostics, and exit statuses. Added invariant VF-C8, focused example and property tests, copied-plugin fresh-install tests for POSIX and PowerShell, repository Python-quality and CI wiring, stricter renderer-output eval checks, documentation updates, and matching plugin version 0.2.0 manifests.

Verification

• bun run check:python — passed; verification backend: 33 tests with 100% line and branch coverage.
• bun run lint, bun run lint:ts, bun run lint:shell, bun run typecheck, bun run check:decisions, and bun run check:docs — passed.
• scripts/test-python-quality-changes and scripts/verify-python-quality-results coverage exercised through the repository Python gate — passed.
• bash plugins/capability/darrow-verification/tests/fresh-install.test.sh — passed.
• /bin/bash plugins/capability/darrow-verification/tests/fresh-install.test.sh — passed.
• claude plugin validate plugins/capability/darrow-verification — passed.
• Skill inspector and all 16 verification dry-run eval cases — passed.
• Live criterion-gap eval: Codex GPT-5.6 Terra medium — passed.
• Live criterion-gap eval: Claude Sonnet 5 medium — passed.

Review notes

The highest-risk surface is cross-platform path handling and exact byte rendering in assessment.py; focused unit, property, copied-install, and eval coverage target those contracts. Native Windows execution is wired into the Python package and fresh-install CI matrices; local host verification was performed on macOS.

Checklist

• I have read and followed CONTRIBUTING.md, including the contribution
  licensing terms.
• I added or updated the applicable invariant before implementation, or
  this change does not affect a capability invariant.
• I added or updated colocated evals, or this change does not affect skill
  behavior.
• I confirmed that each changed plugin remains self-contained, or this
  change does not affect plugin content.
• I ran bun run check:python, or this change does not affect registered
  Python packages or their repository quality infrastructure.