perf(packages): declare side effects and test single-export tree-shaking

[vamgan]

Summary

Add correct sideEffects metadata to Askable packages and single-export consumer bundle fixtures so importing one component/hook does not retain unrelated adapter modules.

Evidence

The React package exports dozens of runtime modules but does not declare sideEffects. A verified esbuild fixture importing only Askable measured:

Import shape	Minified	gzip
Askable from package root	5,825 B	1,534 B
direct dist/Askable.js	329 B	266 B
package graph with temporary sideEffects: false	329 B	261 B

This is a fast, compatible first phase of the broader lightweight-runtime work in #367.

Proposed fix

• Add "sideEffects": false to packages with no import-time registration.
• Treat @askable-ui/web-component separately: split definition from registration or use a precise side-effect allow-list.
• Add representative bundle fixtures for core and every framework adapter.

Acceptance criteria

• Single-export imports tree-shake to a small deterministic bundle
• Web Component registration behavior is preserved
• esbuild and at least one Rollup/Vite or webpack fixture pass
• Gzip budgets run in CI
• Package manifests document intentional side effects
• Work is linked to perf: add a lightweight runtime entry point and bundle-size budgets #367 as the non-breaking first phase