docs: sharpen the open-source adoption path

[MarcoPorcellato]

Summary

• lead the README with the expensive-CI problem, exact-head mechanism, fit
  decision, shortest safe inspection, and bounded dogfooding proof;
• document PR docs: clarify dry-run replay boundary #71 as an exact-anchor case study without identity, savings,
  hosted-parity, or cross-platform overclaims;
• add public support routing and a structured adoption-help source form;
• add an uploadable 1280x640 social-preview candidate plus exact repository
  description/topic proposals;
• add executable contracts for public local links, PNG structure, and issue-form
  schema while leaving human copy under editorial review.

Local evidence

• full locked workspace/all-target/all-feature test suite: PASS;
• all-target/all-feature Clippy with warnings denied: PASS;
• workspace documentation build: PASS;
• public-document contract: 5 passed;
• repository-hygiene contract: 7 passed;
• release-hardening contract: 7 passed;
• release metadata: current;
• branch diff check: clean;
• structured editorial acceptance: PASS;
• broad branch review: no Critical or Important findings;
• preview SHA-256:
  3b4aa8fe0cd8f0122065990133d64f03cf86089dab383b4bd90ec11d4e49ff4a.

Boundaries

• This PR changes no runner/runtime behavior and does not begin M2.
• The repository remains at the documented prerelease state.
• The issue-form source, metadata proposal, and preview candidate are not proof
  of their live GitHub rendering or activation.
• No CCP receipt/evidence for this exact head has been published yet.
• CCP qualification, evidence publication, ready transition, merge, and
  repository settings remain separate gates.