Preserve div pseudo-form binding anchors in WordPress site plans

## Problem

A div-based pseudo-form can produce a valid `html_form_fallback` and generic form entity, but its binding points at synthesized readable block markup that is not present in the converted page. `WordPressSitePlan::assertEntityBindingsRemainPageOwned()` rejects the detached anchor, and `ArtifactCompiler` omits `source_reports.wordpress_site_plan` with `wordpress_site_plan_not_self_contained`.

This was discovered while verifying #700. Real `` elements are unaffected and remain the scope of #700.

## Reproduction

Compile an artifact containing a plain container with a data-entry control and submit-like button, for example:

```html

Inspect the result:

- `fallbacks[0].diagnostic_code` is `html_form_fallback`.
- The fallback has generic controls and a binding.
- `source_reports.wordpress_site_plan` is absent.
- Diagnostics include `wordpress_site_plan_not_self_contained` because the binding search markup is not in the canonical page markup.

## Root Cause

`captureDivBasedPseudoFormFallback()` passes a separately synthesized readable block to `formFallbackFinding()` without replacing or composing the actual converted subtree. The runtime declaration therefore references an anchor the page does not own.

## Acceptance Criteria

- Div pseudo-forms retain an exact page-owned binding anchor in canonical block markup.
- Artifact compilation emits a self-contained WordPress site plan.
- Generic form metadata, controls, and bounded control topology survive unchanged into `generic/forms/v1`.
- Existing real-form behavior and readable pseudo-form output remain unchanged.
- Contract coverage proves both the fallback and site-plan projection.

Start with captureDivBasedPseudoFormFallback() and formFallbackFinding(), then trace WordPressSitePlan::assertEntityBindingsRemainPageOwned() and ArtifactCompiler. Reproduce the provided div pseudo-form and inspect the fallback, canonical block markup, binding, and source report. Done means the binding is page-owned, the site plan is emitted, generic form topology is preserved, and contract coverage proves both projections.

Written by the indexing model from the issue text.