Only fail the settle loop on observed motion, not starvation

[kyleve]

Summary

• Fixes the intermittent CI snapshot failure (Snapshot content never settled from SnapshotRenderingSupport.swift:38) that hit the About suite on cold, loaded runners — including the red run on Drop the stale WhereCore dependency from StuffTestHost #144, which was this flake, not that branch's changes.
• Root cause: under CPU starvation a single settle pass (16ms sleep + layout + quarter-res render) can cost over a second, so the three passes stability needs don't fit the 2.5s budget — and the loop failed static content as "still changing" even though it never once observed a change. Reproduced locally by duty-cycling SIGSTOP/SIGCONT on StuffTestHost (1.4s stopped / 0.12s running); post-timeout frames were byte-identical, and the captures still matched their references.
• Fix: the budget now bounds observed motion. .timedOut requires a change seen past anchor establishment; a change-free loop keeps running until it can prove stability, giving up only at a hard cap of 4× the budget with a new .starved outcome naming the pass count (an environment failure, not view motion). Genuinely moving content still fails at the budget exactly as before.
• Settle failures now name the full snapshot identifier (renderSnapshotImage(of:named:...)), so a timeout in a 16-image matrix says which configuration — the gap that made the CI failures undiagnosable from the result bundle.
• Deadlines measure with ContinuousClock, closing that P2 in the module TODOs.md; docs (README.md, AGENTS.md, TODOs.md) updated in the same change.

Test plan

• New SnapshotRenderingSupportTests pin all three ending conditions: starved-but-static settles (pre-fix code always failed this), observed motion times out at the budget, unsampleable content ends .starved at the hard cap.
• SnapshotKitTestingTests bundle green on the checkout simulator.
• Replayed the exact starvation repro against the fix: the duty cycle that previously produced four settle timeouts now runs the About suite green (103s — every capture extended past its budget, proved stability, and matched its reference).
• Full StuffSnapshotTests scheme green unstarved (no behavior change for healthy runs).
• ./swiftformat --lint clean.

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