Perf: parse scene YAMLs once in generate-batch (discover → select → summary)

[shaypal5]

Context

generate-batch currently parses every matching scene YAML up to three times:

1. _discover_scene_configs() — parses all YAMLs to filter by project/tier
2. _select_configs_by_typology() — parses the selected paths again to read violence_typology
3. _print_selection_summary() — parses again to build the typology count table

Proposed fix

Return SceneConfig objects (or a (Path, SceneConfig) tuple) from _discover_scene_configs and thread them through _select_configs_by_typology and _print_selection_summary, so each YAML is parsed exactly once.

Impact

Low priority: Phase 1 config libraries are small (<50 configs) so the overhead is negligible today. Worth addressing before Phase 3 scale-up (~hundreds of templates).

Flagged from PR #7 review (COPILOT-4).