Store and path generalisation

[forman]

Summary

Generalize RemoteState paths and root state handling

This PR generalizes RemoteState’s path model so the root state can be any value, not only an object-like mapping.

Changes

• Allows empty paths to address the root state:
  • Python: "" / ()
  • TypeScript: "" / []
• Allows paths to start with bracket segments, e.g. [0].label and ["display name"].value.
• Makes Python Store generic over its root state type and exposes store.state.
• Adds notebook-friendly Python item access:
  • store["items[0].label"]
  • store["items", 0, "label"]
  • store[()]
• Defaults read APIs to the root state when no path is provided:
  • Python Store.get()
  • Python built-in Service.get()
  • TypeScript Store.get()
  • TypeScript useRemoteStateValue()
• Simplifies the TypeScript path API by removing the redundant RelativePath export; Path is now the single parsed segment-array type.
• Updates the TS store cache so root subscriptions overlap descendant updates, root updates materialize cached descendants, and leaf updates can patch cached root snapshots.
• Updates README and CHANGES entries for both Python and TypeScript packages.

Checklists

Python package

• pixi run format
• pixi run lint
• pixi run mypy
• pixi run tests
• Updated remotestate-py/CHANGES.md

TypeScript library

• npm run format
• npm run checks
• npm run tests
• npm run build
• Updated remotestate-ts/CHANGES.md

Demo app

• npm run checks
• npm run build
• Updated remotestate-demo/CHANGES.md

[forman]

Ok.