fix(settle): a blank capture never stands in for the tree check

[leeguooooo]

Two fixes. One finding that did not pan out is recorded here so nobody repeats it.

1. Blank frames. An app that hides its screen from capture (PayPay) gives the same blank frame whether or not it moved. Two identical blank frames around the tree read therefore declared a moving screen settled. Blank frames (flat content band, redaction::content_band_is_blank) now fall back to comparing trees. Test: blank_frames_never_count_as_settled.
2. Test fixture. FRAME_B was non-canonical base64 and failed to decode, so frames_that_differ_fall_back_to_comparing_trees was really exercising a failed capture.

Tried, not kept: wait for frames to hold still before the first tree read.
Hardware: v0.13.1, Settings 通用 → 关于本机 → back → back, observe on, USB.

• Baseline: mean 6.1 s per step.
• 2 s cap: 4.7 s on the first run, 5.9 s on the second.
• 4 s cap: 6.0 s.

Per page:

• Plain transitions (通用, some backs) dropped from 6.5 s to about 3 s, with one /source read.
• Pages that load content after the transition (关于本机) went from 4.6–6.8 s to about 7 s. The frames held still, the content changed during the 1.4 s tree read, and a second read followed anyway.

The net change is within noise, so the behaviour is not merged. The bigger lever is the cost of each tree read itself: the wechat-project session found that isVisible dominates /source time (heavy-tree lite-source work).

server + mcp: 598 passed.

[coderabbitai]

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• crates/server/src/http.rs
• crates/server/tests/agent_input_settle.rs

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