Emit computed layout graphs for pseudo-form roots

## Problem

The merged generic form-layout producer treats only literal `` elements as layout-graph roots. `formFallbackFinding()` invokes it for both real forms and div-based pseudo-forms, but `FormLayoutGraphBuilder::collect()` returns before visiting controls when the root tag is not `form`. Styled pseudo-forms therefore emit `generic/computed-layout-graph/v1` with `nodes: []`, no variants, and no diagnostic.

## Work

- Treat the bounded fallback element passed to the builder as the graph root independent of its tag name.
- Preserve control references, wrapper parentage, computed layout, conditional variants, and existing bounds.
- Keep form semantics separate from generic layout topology.
- Emit explicit loss when a pseudo-form root cannot be represented.

## Acceptance

- A styled div pseudo-form emits a root layout node and source-ordered control parentage.
- Literal form behavior remains unchanged.
- Empty graphs cannot silently claim successful layout capture.
- PHP transformer contract and full suites pass.

Found while reviewing merged PR #719 through GPT-5.6 Sol in OpenCode; Chris owns the issue and acceptance criteria.

Start with FormLayoutGraphBuilder::collect() and the formFallbackFinding() call path to compare literal form and pseudo-form roots. Verify control references, wrapper parentage, computed layout, variants, and bounds, then run the PHP transformer contract and full suites; done means styled div pseudo-forms emit non-empty graphs while literal form behavior remains unchanged.

Written by the indexing model from the issue text.