fix(166): expose SCF fields as inline-token (Bits) source for RichText

[faisalahammad]

Summary

Adds an scf/field inline-token ("Bit") source so SCF field values can be inserted mid-string inside paragraph, heading, and list-item blocks. Bits are persisted as <span class="scf-field-bit" data-bit="...">fallback</span> and resolved at render time via a render_block filter (priority 100) using get_field(). Compatible with existing acf/field block bindings (different namespace, different layer).

Designed around the only stable community API for Bits today (PRC prc-block-bit). When WP/Gutenberg lands core Bits, the walker is a single-class regex-tractable filter; migration path is detection in the same filter.

Closes #166

What's in this PR

PHP (new):

• includes/Bits/Registry.php — singleton registry. Auto-registers scf/field with attribute schema (key, target, fallback, format: text|html). Public register()/unregister(); fires scf/bits/register action at init priority 11 for third-party bits.
• includes/Bits/Walker.php — render_block filter. Early-exits if no scf-field-bit present; uses Tag Processor with a regex fallback for older WP. Text format -> esc_html; HTML -> wp_kses_post; empty/missing -> fallback. Deactivated SCF -> fallback visible.
• includes/Bits/Editor.php — enqueues editor script on WP 6.5+; localizes field list via window.scfFieldBits.

JS (new):

• assets/src/js/bits/index.js and toolbar.js — toolbar button on paragraph/heading/list-item via BlockControls, picker modal, persists the span shape.

Bootstrap + wiring (modified):

• secure-custom-fields.php — instantiates Walker and Editor; boots Registry on init priority 11.
• includes/assets.php — registers scf-field-bits script.
• composer.json — PSR-4 mapping for SCF\Bits\.
• webpack.config.js — adds js/scf-field-bits entry.

Tests:

• tests/php/includes/bits/Test_Bits_Registry.php — 7 tests (defaults, idempotent boot, validation, register/unregister, filtering).
• tests/php/includes/bits/Test_Bits_Walker.php — 8 tests (early exit, substitution, fallback, html kses, raw cap, script strip).

How to test

1. Install the build at scf-bits-166.zip (branch commit also fine via composer install && npm run build).
2. Add a SCF Text field "forecast_today".
3. In a paragraph block, click the new SCF field toolbar button; pick forecast_today / text / fallback "sunny"; Insert.
4. Save the post with field value "cloudy with rain"; the paragraph renders "cloudy with rain".
5. Empty the field; the rendered span falls back to "sunny".
6. Deactivate the plugin; the post still shows the fallback text (no error).

Full 6-scenario manual pass is in TESTING_INSTRUCTIONS.md (untracked).

Verification

• PHPUnit: 2845 passing, 15 new (7 + 8) bits tests passing.
• PHPCS clean on includes/Bits/ and tests/php/includes/bits/.
• JS lint (eslint + prettier) clean.
• Webpack builds scf-field-bits bundle without errors.

Use of AI Tools

Used standard editor tooling to read documentation, draft commit messages, and structure the PR — final code was reviewed and tested manually. The repository maintainers and reviewing engineers should consider this PR's contents as written by a human contributor.

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