fix(templates): expose custom data tokens globally

[DavidBabinec]

What changed

• expose rows from non-system custom data tables in the binding picker on ordinary pages
• persist bindings as {data.<rowId>.<fieldId>} and resolve them in canvas previews, static publishes, holes, and infinite-loop fragments
• prefetch only referenced custom rows and snapshot their current live cells during Site publish
• document the binding source and add picker, token-engine, repository, and public-rendering regressions

Why

Custom data tables are the natural home for shared values such as a GitHub star count, but their rows were not available outside loop or post-type template scopes. Plain custom-data rows are also deliberately non-versioned, so the publisher must read their live cells rather than look for an active published row version.

User impact

Authors can insert a custom-data value into any compatible property and reuse it across ordinary pages. The canvas and published site resolve the same value; a later data change is included by the next Site publish.

Verification

• bun test (6,621 pass)
• bun run build
• bun run lint
• bunx tsc -b --pretty false
• disposable local browser E2E: create table/row, insert token on homepage, verify all canvas frames, publish, verify anonymous homepage