fix(effects): address the review of the shader effects integration

[EtienneLescot]

Summary

CodeRabbit's eight comments on #1210, all on the shader effects rows.

• Loop counting (Major). A counter now starts from its value at that loop: the last store before it in its block, else its initialiser. Any other write to the counter (another block, after the loop, through a pointer, a second loop on the same counter) is refused. The comment's example only showed a false refusal; the same gap let if (…) { i = -100000; } for (; i < 4; i++) count 4 trips for 100 004, under the 4096 safety ceiling.
• Cache eviction. The compiled-effect cache never evicts an effect the current scene shows (shader_fx::eviction_victim, Windows and Linux). set_scene stores the scene before warming the cache so eviction can see it. feat(compositor): the macOS shader effects pass (Metal) #1206 (macOS) will want the same.
• setEffect clamps a moved pill against its own lane only, the one the overlap check used.
• checkShaderEffect returns null ("not checked") when the addon throws.
• Workbench. shaderEffect is an EditFamily; the ripple scenario counts only ranges of an effect that reads ctx.clicks.
• Docs. checkShaderEffect in the spec (E.6 now says the in-app agent has the checker); effects in ai-agent.md's removeModifier row and time-base note.

Related issue

None.

Type of change

• Bug fix
• Documentation

Release impact

• No release note needed

Desktop impact

• Windows
• Linux
• macOS: shared code only

Testing

• Rust: new the_counter_starts_from_its_value_at_the_loop and eviction_spares_the_effects_the_scene_shows. Windows cargo test -p openscreen-compositor -p compositor-view-napi --lib: 448 + 1 passed. Linux on lavapipe under WSL: see the comment below.
• TS: new moves a region against its own lane only, which fails on the old code (start clamped to 6 s instead of 4 s). agent-tools.test.ts and electron/mcp: 318 passed.
• npx tsc --noEmit, -p tsconfig.test.json, -p tsconfig.workbench.json, Biome on the touched files.

🤖 Generated with Claude Code

[EtienneLescot]

Linux, on lavapipe under WSL, at ee5e02e: cargo test -p openscreen-compositor --lib --tests, 483 passed plus the integration tests.

[coderabbitai]

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