docs: clarify core runtime public surface

[smiggleworth]

Closes #335

Summary

• replace actor-backed framing with the actual single-threaded lane-scheduled model
• define /data as canonical while documenting root query exports as compatibility conveniences
• show and explain both equivalent state() forms in the README and state/data guides
• align the package description and root source header with the documented model

Verification

• fresh build and all 37 repository checks passed
• formatting, lint, and typecheck passed
• local broad jsdom execution ran 1,443 tests green before host clock jumps prevented nine workers from starting; hosted CI is the complete merge gate

The package version remains 0.2.2. No lockfile, release artifact, tag, or publication was changed or created.

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