Add section variants and scan enhancements to phase-0 PRD

[smorin]

Summary

This PR adds two major feature specifications to the phase-0 PRD: Section Variants (paired and grouped sections) and Section Scan Enhancements (variant-aware scanning and validation).

Key Changes

Section Variants (0.13)

• Introduces colon-separated syntax for variant groups (group:variant) to enable toggling mutually exclusive code blocks
• Defines CLI behavior for solo sections, paired variants, and multi-variant groups
• Adds --pair enforcement flag to validate exactly 2 variants in a group before execution
• Specifies multi-file variant toggling with cross-file consistency checks

Section Scan Enhancements (0.14)

• Extends --scan output to be variant-aware, grouping variants under parent groups
• Adds detailed section view with -S filter to show file references per variant
• Introduces --check validation mode for read-only integrity checks (matching markers, variant consistency, duplicate detection)
• Adds --json output support for machine-readable scan results
• Documents new CLI flags: --pair, --check, and -S with --scan

Notable Details

• Variant markers use existing toggle:start/toggle:end syntax with no new keywords
• Backward compatible: solo sections (no colon) retain unchanged behavior
• Validation includes cross-file consistency warnings and pair-count enforcement
• JSON schema provided for programmatic consumption of scan results

https://claude.ai/code/session_01UDBD76xiXGToxAeFbxSWzT