[codex] Rename evolution storypoints

[jhull]

Summary

• Rename the MC/OC perspective storypoints from Main Character Evolution and Influence Character Evolution to MC Steadfast Expression and OC Steadfast Expression in the canonical schema enum.
• Keep the JSON schema and YAML schema twins in sync.
• Update the complete storyform template, terminology reference, and history note to use the new labels.

Why

The older MC/OC Evolution storypoint labels now collide conceptually with the Character Evolution Audience Experience Dynamic added in Fall 2025. The new MC Steadfast Expression and OC Steadfast Expression labels preserve the intended perspective-storypoint meaning while keeping Character Evolution available as the Audience Experience Dynamic.

Impact

Consumers validating canonical appreciations should emit the new MC/OC storypoint names. Existing uses of the Audience Experience Dynamic Character Evolution remain valid and unchanged.

Validation

• node tests/validate-schema.js -> Schema validation checks passed (7 valid + 9 invalid fixtures).