CmdPal: Icons (9/n) - Prevent stale recycled icons and reuse image presenters

[Jiří Polášek (jiripolasek)]

Summary of the Pull Request

Part 9 of 12 in the CmdPal icon-loading series. Depends on #50188. Next: #50190.

This PR clears previous-item presentation immediately and reduces image-control churn.

• Adds placement-owned fallbacks and resets resolved/request-specific fallback state when the source changes.
• Alternates inactive Image slots in a stable Grid instead of rebuilding the image subtree.
• Keeps app-image fallback policy separate from valid glyph/emoji hero rendering.

Motivation: a recycled row must not show another item's icon while loading, and repeated image updates should reuse presentation where WinUI permits it.

Evidence

Historical adjacent-stage comparison (2026-08-18); shared method and limitations: the diagnostics foundation PR #50181.

• C cold-ish icon-element update average: 0.188 → 0.074 ms (−0.114 ms); all three pairs decreased.
• C cold-ish elements created: 454 → 84; elements reused: 0 → 382.
• C cold-ish cumulative measured icon UI time: 119.588 → 67.655 ms (51.933 ms less across an approximately 28-second search scenario).
• C cold-ish applied average: 8.402 → 8.483 ms (+0.081 ms), inconsistent across pairs; no request-latency benefit is established.
• Presentation-state tests cover reset and asynchronous fallback transitions. Final visual recycling still needs a UI smoke test.

The measurable benefit is less control construction and cumulative UI work, alongside the stale-icon correctness fix. A 0.114 ms update saving is not a claim that the app feels 61% faster.

Technical notes

• Two lazily created image slots are an intentional workaround for observed blank/stale results when reusing one presenter. They are not a claim that all WinUI icon elements can safely be reused.
• Request-specific fallback state must be reset together with the resolved source. Clearing only the displayed source can retain the previous item's fallback.
• Recycled controls must not explicitly dispose shared cached image resources; those may still be used by other controls or deferred WinUI copies.
• Ordinary glyphs and emojis remain valid hero sources. The shared bitmap placeholder is an app-icon policy, not a blanket ban on glyphs in details.

Implementation: src/modules/cmdpal/Microsoft.CmdPal.UI/Controls/IconBox.cs.

PR Checklist

• Closes: CmdPal: Icon loading (8/n) - Fix recycled icon presentation and reuse presenters #49941

• Communication: I've discussed this with core contributors already. If the work hasn't been agreed, this work might be rejected
• Tests: Added/updated and all pass
• Localization: All end-user-facing strings can be localized
• Dev docs: Added/updated
• New binaries: Added on the required places
  • JSON for signing for new binaries
  • WXS for installer for new binaries and localization folder
  • YML for CI pipeline for new test projects
  • YML for signed pipeline
• Documentation updated: If checked, please file a pull request on our docs repo and link it here: #xxx

Detailed Description of the Pull Request / Additional comments

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