docs: add storage backend blueprint landing pages

[bjacobso]

Triplex's site explains custom runtimes but has no backend-specific path from storage primitives to a working adapter. Add landing pages at /sqlite, /foundationdb, /datomic, and /xtdb, plus a /backends index linked from the homepage, navigation, and custom runtime guide.

Each blueprint maps storage, transactions, temporal reads, journal/receipts, and query execution to the Triplex contract, with a concrete correction scenario and implementation or conformance steps. SQLite is labeled supported, FoundationDB experimental/private, and Datomic and XTDB proposed with no existing adapters. The proposals explicitly cover the mismatch between Triplex's Effect transaction callback and native declarative/non-interactive writes. The homepage now describes current XTDB's SQL/XTQL model accurately.

Includes shared responsive styling, checked SQLite and FoundationDB examples, agent-readable Markdown exports through the existing documentation pipeline, and refreshed homepage visual snapshots.

Validation:

• pnpm check passed (formatting, lint, typechecks, checked docs, default tests, package and docs builds).
• pnpm test:e2e passed: 13 tests, including the refreshed light/dark homepage snapshots.
• Final production docs build passed; all five routes and Markdown/LLM exports checked in light/dark mode at desktop and mobile widths, including status labels, anchors, valid exported card links, and no page overflow or errors.
• Executed the SQLite example and verified persistence, duplicate-command rejection, and recovery of the original receipt.

Documentation only; no storage adapters are added or promoted. FoundationDB's example is type-checked but native integration was not run.

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