Compositions phase 6: docs pages and CLI-vs-plugin parity coverage

[nathanacurtis]

Phase 6 of #622. Closes out the project: docs for every touched command, and parity coverage.

Docs (specs repo, site/src/content/docs/cli/commands/)

• scan.md — Compositions section: listing rule (only READY_FOR_DEV frames; unmarked frames are never listed; the marker qualifies wherever the frame lives — pages, sections, other containers), curated checkboxes, rescan merge semantics, --include-all not applying to unmarked frames
• generate.md — composition output location and layout-flag behavior; single-component mode with a frame node ID; bridge mode already covered (frame selection) — reconcile wording
• react.md / webcomponents.md — compositions output tree, single story, Compositions nav group, Pro-only gating (skipped with a note on free)

Parity (specs-testing)

• Compare the CLI composition spec against the plugin ground truth for the fixture frame (detect level)
• Verify the emitted composition renders in Storybook (bring it up with scripts/storybook-up.sh; on an index failure run the index guard, don't restart)
• Test-round artifacts belong under specs-testing/tests/NNNN/

Acceptance criteria

• All four docs pages cover compositions, consistent with shipped behavior and with each other
• Parity coverage includes the composition fixture and passes
• The composition story renders in Storybook for both targets