feat(observe): an observed action with no baseline diffs against the owner's last read

[leeguooooo]

Why: in the usage log (2274 requests over about two days), 0 of 821 element reads used ?since, and 23 of 1010 actions asked for return=delta, most of them our own benchmarks. Plain HTTP agents never carry the snapshot token, so the diff feature went unused.

What changes:

• The daemon remembers the last snapshot each named owner (X-Phone-Owner) was handed: an elements read, an observed action, or an observed batch.
• An observed action that names no baseline (no since, no snapshot in the body) now diffs against that snapshot.
• An explicit since or snapshot still wins. Anonymous clients are unchanged.
• The map holds at most 64 owners.

Hardware measurement on v0.13.1, Settings: 通用 → 关于本机 → back → back, 2 rounds per mode.

mode	daemon calls	bytes the agent reads	daemon time
A: action, then a full /agent/elements	16	155 KB	21.9 s
B: observed action (return=delta)	8	50 KB	44.4 s

What the numbers say:

• Observe halves the calls (each saved call is a model turn) and cuts what the agent reads by 68%.
• On a heavy tree it is slower on the daemon side. Settle compares tree reads (each /source is 1.3–1.7 s for ~400 KB) whenever the frames around the first read differ mid-animation. A follow-up will settle on frames first and read the tree once.
• A whole-screen change gives a delta close to a full tree (16.9 KB for 关于本机), as expected.

Tests: an_observed_action_diffs_against_the_owners_last_read. server + mcp: 597 passed.

[coderabbitai]

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