docs: compare JPA cascades with Storm write sets (reference + tutorial)

[zantvoort]

Summary

• New reference page JPA Cascades vs Write Sets (docs/jpa-cascades-vs-write-sets.md, sidebar: Resources). Implements the same Owner-Pet-Visit graph in both models and explains the philosophy: cascades travel parent-to-child along mapped collections, write-set discovery follows foreign key fields upward, so insert discovery needs no configuration and delete discovery deliberately does not exist. Includes an action-by-action mapping (persist/merge/remove) and a cascade translation table. Cross-linked from the write-sets reference and the JPA migration guide.
• New tutorial Persisting object graphs: cascades vs write sets at /tutorials/object-graphs, added to the From JPA series with its hub card.
• The referenced-entity contract, stated with its rationale in the WriteSet javadoc, the write-sets reference, and both new pages: a write set writes the entities you name, plus the entities your values make necessary. A modified keyed entity held in a foreign key field is not written (a hydrated snapshot is not write intent); pass it as an explicit member.
• Write-set operation terminology unified on "action" (was "verb") across javadoc, KDoc, docs, and the private enum in WriteSetImpl; the write-set test suite passes (25 tests).
• Site polish: one gold family (#feeeb0 / #fbbf24 / #f59e0b) across the figure gradients and accents, centered benchmark matrix headers, styled inline code inside tutorial note boxes (previously fell through to Infima's light styling), and the benchmarks page now uses the example pages' clone bar with a "View on GitHub" button.

Notes

• The write-set snippets use the vararg call shape (writeSet().insert(wolfie, rex, visit)), which does not exist on WriteSet yet; the vararg overloads need to land before the 1.13.0 release ships these pages.
• The reference pages are docs/-only and appear at /docs/next until the next docs snapshot; the tutorial deploys with the site. Merging around the release keeps the tutorial's /docs/write-sets links from going live before the target pages do.

Verification

• Full Docusaurus production build passes; both pages render (tabs, editor panels, hub card, series count).
• storm-core write-set tests pass after the terminology rename (25 tests).
• Copy checked against site rules: no em dashes, no banned words, JPA treated neutrally (cascade convenience acknowledged; trade-offs stated symmetrically).