Add deterministic conditional page injection to FormSpec generation

[danielnaab]

Goal

As a person filling out a digitized form, I want pages that don't apply to me to be skipped automatically, so I don't waste time on irrelevant questions.

Context

Story #121 improved FormSpec layout quality by +19.8pp through a layout-aware extraction prompt. However, conditional page use (scoring pages as skippable based on user answers) improved only modestly (37.5% → 43.8%) despite explicit prompt instructions and worked examples.

The core difficulty: the LLM must derive page-level conditions from field-level conditions in the DataCollectionSpec. This requires:

1. Identifying groups where most/all requirements share a common condition
2. Separating the "gate" question (the field that triggers the condition) to a prior page
3. Adding the correct condition JSON to the page

This inference is unreliable via prompt alone. A deterministic post-processing step would be more robust.

Scope

Implement a post-processing function that runs after FormSpec generation (Step 2) and adds condition properties to pages where appropriate:

• Scan each page's groups for fields with shared conditions
• If ≥80% of a group's non-gate fields share the same condition, lift it to the page level
• Ensure the gate field is on a prior page (reorder if needed)
• Validate the resulting FormSpec still passes schema checks

Acceptance Criteria

• A addPageConditions(spec: DataCollectionSpec, formSpec: FormSpec) => FormSpec function exists
• It correctly identifies and adds page-level conditions for at least 2 of the 4 test fixtures
• Layout quality evaluation shows conditional use metric ≥ 62.5% (score 3.5+/5 average)
• Existing tests continue to pass
• The function is wired into the extraction pipeline as an optional post-processing step

Related

• Story Optimize generated form layout to follow best practices #121 (parent work)
• catalog/experiments/layout-quality/findings.md — documents the prompt-only ceiling
• src/services/form-documents/layout-prompt.ts — current prompt with conditional guidance