feat: establish Topcoat Blocks with a portable sign-in form

[superposition]

Why

Create an independent, copy-to-own UI block library for Topcoat with a visual and semantic feedback loop that does not use the upstream Topcoat repository as a staging area.

What ships

• a custom topcoat-blocks registry crate;
• a portable POST-based sign_in_form composed from Topcoat button, card, input, and label primitives;
• optional password-recovery and sign-up paths;
• a child slot for CSRF and other hidden form fields;
• a copy-installed Coatcheck workbench with default/minimal stories across five responsive surface scenes;
• contribution, acceptance, roadmap, and CI policy.

Publish boundary

The registry is the product. Coatcheck, Playwright, Bun, and the workbench remain repository-only tooling. scripts/release-smoke verifies the crate contains the registry source and rejects review-harness files; the package currently verifies as 7 files / 8.2 KiB.

Validation

./scripts/check passes locally:

• Rust formatting, workspace tests, and Clippy with warnings denied;
• clean cargo package verification and harness-isolation smoke;
• 13 browser tests;
• 10 generated visual cases: default and minimal at mobile/tablet/desktop, light and dark;
• zero Axe violations at every impact across all cases;
• zero browser console or page errors;
• keyboard order, native Enter submission, label ownership, autocomplete, required fields, POST action, optional links, unique IDs, and hidden-field composition.

Selected review evidence

Mobile light	Desktop light	Desktop dark

Commit train

1. chore: scaffold the topcoat-blocks registry
2. feat: add the Coatcheck workbench
3. feat: add portable sign-in form
4. docs: define the block acceptance gate

Deliberately deferred

• no crates.io release in this PR; Git dependency installation is documented;
• no upstream Topcoat proposal until a component proves primitive-level reuse here;
• sign-up, recovery, shell, and dashboard blocks remain sequenced in the roadmap rather than opened as speculative implementation work.