Polish SongForge structure and exports

[WalksWithASwagger]

Summary

• Add deterministic structure variants and originality guardrails to SongForge packs.
• Render guardrails, structure variants, prompt packs, and source notes cleanly in markdown/vault exports.
• Update fixture evals and presentation docs to keep SongForge positioned as source-linked text workflow, not direct music generation.

Verification

• PYTHON=/Users/kk/Code/spektorAI/venv/bin/python make lint
• PYTHON=/Users/kk/Code/spektorAI/venv/bin/python make test -> 283 passed
• PYTHON=/Users/kk/Code/spektorAI/venv/bin/python make eval-fixture
• git diff --check

Closes #52

[github-actions]

Agentic PR Review

Verdict: needs-human

Checks:

• Linked issue via Closes #...: yes
• Issue acceptance criteria found: 4
• PR verification section present: yes
• PR acceptance self-check present: no
• PR self-check covers issue acceptance: no
• Diff within v1 limits: yes
• Missing acceptance coverage:
  • SongForge emits multiple structure variants with source notes and originality guardrails.
  • Markdown/vault exports preserve lyric, spoken-word, prompt-pack, and source-note sections cleanly.
  • Presentation docs describe SongForge as a source-linked text workflow, not direct music generation.
  • Fixture tests pin the output structure.

This PR is not blocked from human review, but the agentic gate is not satisfied.