docs(launch): the pre-launch gate and the announcement plan (#291)

[eaitbrahim]

What & why

Closes #291 (with #320, the scans half, already merged). docs/launch.md is the gate the issue asks for — nothing is announced until every box below is ticked — with every box ticked and evidence-linked: Phase 6 (licence/discoverability/positioning), Phase 7 (README, CONTRIBUTING, CoC, templates, Discussions, good-first-issues), the fiqh basis (#317), the review-path stance (#318), the Arabic entry point (#319), CI green on main, the tokenless scans actually configured (#320), and the maintainer-response commitment stated in CONTRIBUTING.md. Below the gate: the audience in the issue's order (Islamic fintech practitioner networks → r/islamicfinance + Muslim dev communities → IIUM/INCEIF/Durham → only then HN/Reddit/Lobsters), what the announcement must say — the measured result in the post, with the real numbers — and a ready-to-adapt draft. Plus the explicit non-goal (not for stars).

Two corrections of the issue's own text, made in the document: the measured result is 0 of 90 and 0 of 82 under production-faithful execution (the "0 of 20" in the issue misremembers the earlier 0-of-19 hourly record), and the scans item is satisfied tokenless (#320) rather than by waiting for SONAR_TOKEN/SNYK_TOKEN. One gate item needed a repo change: CONTRIBUTING.md now carries an honest-for-one-person response commitment (triage 3 days, first PR review a week, security routed to SECURITY.md's SLA).

The announcement itself is deliberately NOT part of this PR — the gate arms it; walking through the door is a human action.

Tests-first evidence

tests/test_launch_gate.py written first: red 8/8 (no doc). Green: 8 passed — pinning the gate rule verbatim, each gate item named, the real numbers + the record link (path-aware: the doc lives in docs/, the link is doc-relative) + file existence, the audience ORDER by position, the non-goal, the draft's load-bearing content (compliance-engine lead, 0 of 90, not a fatwa engine, No scholarly review), CONTRIBUTING's solo-maintainer commitment, and the README documentation-map link.

Gates

• uv run ruff check keel tests packages — All checks passed!
• uv run mypy — Success: no issues found in 237 source files
• uv run pytest -q — 2856 passed, 1 skipped

Scope check

• No rail, rule, or default classification is touched — documentation and pinning tests only.