Prepare Yieldstar 0.5.0 release

[djgrant]

Stack

This PR targets dual-ci and is stacked under #29. It should be retargeted to main after #29 merges.

Summary

This prepares the complete Yieldstar 0.5.0 package matrix for release without publishing anything:

• Confirms all workspace packages at version 0.5.0 and documents the full SQLite-driver, de-Bun, and dual-runtime series in the changelog.
• Adds a complete 0.4→0.5 migration guide covering package renames, SQLite connector selection, worker-invoker changes, synchronous store updaters, and Node support.
• Restructures documentation around runtime families × connectors, with resident-process pages for Bun and Node, and positions Yieldstar as running on both runtimes.
• Hardens release preparation with an inspectable bin/release.sh --dry-run path, explicit public/private package boundaries, Node >=22.6 engine requirements, and repository metadata.
• Marks @yieldstar/test-invoker, @yieldstar/test-runtime, and @yieldstar/test-utils private so the broad scoped-package publish filter cannot release internal test helpers.

Validation

• bun run test:bun: 124 passed, 0 failed, 1 pre-existing skip
• pnpm run test:node: 124 passed, 0 failed, 1 pre-existing skip
• bun run bundle: passed
• bun dts: passed
• bin/release.sh --dry-run: passed and selected only the five intended public packages

No npm publication or live release command was performed.