perf: suppress redundant icon observer callbacks

[ptstory]

Summary

Narrow the icon observation path so updateIcons() only fires when icon-relevant state changes.

The existing render signature inside applyIcon() already avoided redundant icon rendering, but it did not prevent redundant observer work from reaching updateIcons(). This change reduces observer callback and updateIcons() churn before render-signature checking is reached.

Split from #1073 per review feedback. Related: #678.

Not included

• No background refresh changes (see perf: background credits and dashboard fetch for regular refreshes #1078)
• No switcher first-click changes (see fix: accept first click in Codex account switcher #1079)
• No unrelated cleanup from perf: fix status item icon observer CPU loop, background web scrapes, fix switcher first-click #1073

Problem

StatusItemController.observeStoreIconChanges() watches UsageStore.iconObservationToken, which registers Swift Observation dependencies on multiple store fields.

isRefreshing toggles on every refresh cycle regardless of icon state. That causes the observer to re-fire and call updateIcons() even when no icon-relevant state changed.

That does not necessarily mean every observer fire performs an expensive render: applyIcon() already has render-signature checks and often skips unchanged icon rendering. The issue fixed here is narrower: unnecessary observer/updateIcons work was still happening before those render-signature checks were reached.

Changes

• Remove isRefreshing from iconObservationToken because refresh start/finish state is not itself icon-relevant.
• Add storeIconObservationSignature() to derive a signature from actual icon-relevant state.
• Guard the icon observer callback so it skips updateIcons() when the derived store icon signature is unchanged.
• Add lightweight [perf] instrumentation gated behind debugLogLevel=verbose for future diagnosis.

Measurement

Measured with [perf] instrumentation gated by debugLogLevel=verbose.

Scope: startup refresh cycles only.

Metric	Before (main)	After (this branch)
updateIcons() calls per refresh cycle	9–11 (avg 10)	5–6 (avg 6)
Actual renders per refresh cycle	1	1–2
applyIcon() render-signature skips per refresh cycle	10–12 (avg 11)	6–8 (avg 7)

Scope note

This PR does not claim to fully explain or fix the original 99% CPU spike from #1073. After local account pruning from 7 managed Codex accounts to 3 kept/paid accounts, I can no longer reproduce the sustained CPU peg on demand.

The scoped claim here is narrower: refresh-state churn was causing observeStoreIconChanges() to call updateIcons() even when the icon-relevant store signature had not changed. This PR removes that redundant observer work before render-signature checks are reached.

Testing

• StatusItemIconObservationSignatureTests
  • verifies signature stability across isRefreshing churn
  • verifies signature stability across status description/timestamp churn when the status indicator is unchanged
  • verifies signature changes when the status indicator changes
• swiftformat
• swiftlint --strict
• make check

[steipete]

Reviewed the icon observer suppression path, ran focused tests, lint/check, Codex autoreview, and a release bundle relaunch smoke. Added the maintainer changelog entry and rebased onto current main.