fix(plugins): reject mismatched filter results

[DavidBabinec]

Fixes #394

What changed

• Reject filter results whose runtime value type differs from the current pipeline value.
• Keep the previous value, identify the offending plugin in the error log, and continue later filters.
• Document the runtime filter contract for plugin authors and the publisher.

Why

A plain JavaScript publish.html filter could return null, bypass the static bake, and make the live renderer serve an empty 200 response. Validation belongs in the hook bus so every filter pipeline gets the same fallback and plugin-aware diagnostics.

Verification

• Before: targeted regression test failed, 10 pass / 1 fail
• Control without source fix: failed with the same null-good result
• bun run build (exit 0)
• bun run lint (exit 0)
• bun test (exit 0, 6,726 pass / 0 fail)