refactor(core): harden observation evidence pipeline

[quanru]

Summary

• make observation evidence an atomic typed model of screenshots and source-frame indices
• move prompt and evidence orchestration out of the generic insight extraction flow
• cap visual analysis at 50 evenly sampled source frames and honor cancellation
• preserve subtle UI evidence with lossless intermediate rendering and readable two-line labels
• make sequence prompts, documentation, warnings, and dump serialization match the bounded-evidence contract

Review findings addressed

• corrected prompts that implied every captured frame was sent to the model
• replaced synchronized optional arrays with atomic evidence entries
• replaced broad screenshotSequence* serialization filtering with exact transient fields
• bounded preprocessing work and propagated abort signals
• removed repeated JPEG encoding before final grid generation
• added direct verification that the orange measurement outline is present

Validation

• pnpm --filter @midscene/core exec rstest run — 2,064 passed, 8 skipped
• focused observation regression suite — 16 passed
• pnpm --filter @midscene/core run build
• Biome and git diff --check

Status

Draft/WIP while the prepatch release is being validated.

Prepatch release

• @midscene/core version: 1.13.4-beta-20260928024128.0
• npm: https://www.npmjs.com/package/@midscene/core/v/1.13.4-beta-20260928024128.0
• workflow: https://github.com/web-infra-dev/midscene/actions/runs/36370657854

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[quanru]

跟进 observation evidence 改造。当前相对 feat/observation-report-sequence 无冲突，附着的 20 项检查通过，仍保留 Draft。PR 描述中有 core 定向测试、完整 core 测试和预发布版本。

合并前还需要确认 base 分支的落地安排，以及预发布在真实 observation 使用场景中的验收结果。请维护者先复审原子 evidence 表示、50 帧上限与取消传播的设计；本轮不将单元测试等同于真实模型效果验证。