release: py-shiny v1.6.0

[schloerke]

Summary

Release candidate for py-shiny v1.6.0.

• Bumps CHANGELOG.md [UNRELEASED] → [1.6.0] - 2026-03-20
• No git-based dependencies in pyproject.toml
• All 51 shinylive examples tested and passing

Changelog highlights

• OpenTelemetry integration with configurable collection levels (SHINY_OTEL_COLLECT), @otel.suppress/@otel.collect decorators, and example app
• New toolbar component (ui.toolbar() and related functions)
• New ui.input_code_editor() with syntax highlighting via prism-code-editor
• Reduced package size by ~400KB
• Various reactive/OTel improvements

Test plan

• All 51 shinylive examples pass (automated Playwright test)
• CI passes on this PR
• Manual verification of changelog content

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