chore: isolate story examples from test-support

[huacnlee]

Description

Running story examples enables the gallery's gpui-kit/test-support development dependency. GPUI's App::flush_effects() then synchronously draws dirty windows during updates, adding work to interactive execution.

Move all eight story examples into standalone example-* workspace packages that depend on the gallery normally. Their GPUI dependency graphs exclude test-support and leak-detection. Move fixtures with them, update shared fixture references, and update the documented commands. Editor behavior and story test configuration are preserved.

This is a temporary workaround for the reported Linux stuttering, not a fix for GPUI's test-support performance. Why the slowdown is particularly severe on Linux still needs profiling and an upstream investigation; no before/after frame-time measurements or cross-platform performance verification are claimed here.

Break Changes

Launch the migrated examples by package name:

- cargo run --example editor
+ cargo run -p example-editor
- cargo run --example stream_markdown
+ cargo run -p example-stream-markdown

The same convention applies to brush, dock, html, large-text, markdown, and tiles.

How to Test

• Build all eight packages with cargo build -p example-editor -p example-brush -p example-dock -p example-html -p example-large-text -p example-markdown -p example-stream-markdown -p example-tiles.
• cargo test -p example-markdown: 10 tests.
• Inspect each package with cargo tree -p example-editor -e features -i gpui-pre (substitute each package): no test-support or leak-detection features.
• Rust formatting, diff whitespace, fixture paths, and source-move equivalence checked.

Interactive Linux scrolling and macOS/Windows performance have not been tested.

Checklist

• Read the contributing guidelines.
• Reviewed the migration diff; AI-assisted implementation.
• Ran interactive examples and verified scrolling.
• Tested macOS, Windows and Linux performance.