Add card manifest deck fast path

[walker1211]

Summary

• Add a --card-manifest input path that builds deck JSON directly from a structured article manifest.
• Preserve the existing electronic-pickles and AI deck generation paths when no manifest is provided.
• Add deterministic text fitting, source markers, manifest-relative image hydration, and regression tests.

Test plan

• go test ./...
• go build -o ./mark2note ./cmd/mark2note
• git diff --check