Clarify BFB conversion boundaries

[chubes4]

Summary

• Expose a BFB-side h2bc capability discovery seam that consumes html_to_blocks_capabilities() when h2bc provides it and otherwise reports the tracked h2bc dependency instead of stale placeholder data.
• Keep BFB declared-format only by removing the unused detect() requirement from the adapter interface/docs and preserving explicit bfb_convert( $content, $from, $to ) routing.
• Clarify Site Editor marker ownership as a shared BFB/h2bc extension contract and add BFB-level marker conversion coverage through the public conversion path.

Closes #121.
Closes #122.
Closes #123.

Upstream dependency: chubes4/html-to-blocks-converter#418

Verification

• homeboy --force-hot lint block-format-bridge
• php tests/smoke-capabilities-abilities.php && php tests/smoke-h2bc-capability-discovery-prefixed.php && php tests/smoke-h2bc-capability-discovery-standalone.php && php tests/smoke-multi-consumer-bundled-load.php && php tests/smoke-cli-command.php && php tests/smoke-normalization-api-branches.php && php tests/smoke-content-normalization.php && php tests/smoke-insert-conversion-measurement.php && php tests/smoke-duplicate-version-registry.php && php tests/smoke-scoped-h2bc-hooks.php && php vendor_prefixed/chubes4/html-to-blocks-converter/tests/smoke-site-editor-marker-transforms.php

Notes

• homeboy test block-format-bridge is currently blocked before PHPUnit runs because the active Homeboy/WP Codebox test harness cannot load @automattic/wp-codebox-core or HOMEBOY_WP_CODEBOX_CORE_MODULE for buildWordPressPhpunitRecipe.
• BFB now has the consumer seam for h2bc capabilities, but full real transform inventory still depends on h2bc exposing the public capability API tracked in h2bc #418.

AI assistance

• AI assistance: Yes
• Tool(s): OpenCode (openai/gpt-5.5)
• Used for: Investigating the BFB/h2bc boundary issues, drafting and implementing the BFB capability seam, docs cleanup, tests, and PR description; Chris remains responsible for review and acceptance.