Budget incremental reflection probe updates

[LisaScheers]

Description

Adds a token-bucket CPU budget for incremental, non-realtime reflection probe face updates.

The scheduler measures actual update time, smooths the observed cost, accrues a configurable allowance each frame, and caps idle credit. It forces progress after 30 deferred frames so probe generation cannot stall indefinitely.

RenderReflectionProbeUpdateBudget defaults to 1 ms per frame. Setting it to 0 restores the previous unthrottled behavior. Explicit realtime probes retain their existing six-face-per-frame behavior.

A native Apple Silicon profile of the closely related Firestorm viewer placed LLReflectionMapManager::update() / doProbeUpdate() on roughly 8% of main-thread samples even with static-only reflections. The official develop branch contains the same one-face-per-frame incremental scheduler.

Related Issues

• Please link to a relevant GitHub issue for additional context.

Issue Link: None; opened as a draft for upstream performance evaluation.

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Checklist

• I have provided a clear title and detailed description for this pull request.
• If useful, I have included media such as screenshots and video to show off my changes.
• The PR is linked to a relevant issue with sufficient context.
• I have tested the changes locally and verified they work as intended.
• All new and existing tests pass.
• Code follows the project's style guidelines.
• Documentation has been updated if needed.
• Any dependent changes have been merged and published in downstream modules.
• I have reviewed the contributing guidelines.

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Additional Notes

Validation performed:

• clean git diff --check
• xmllint --noout indra/newview/app_settings/settings.xml
• exact stable patch-ID match with the Firestorm implementation
• reviewed credit accrual, measured-cost debit, disabled-budget behavior, and starvation escape

A full official viewer build was not available because this host lacks the autobuild dependency bundle and full Xcode toolchain. This remains a draft pending native build and benchmark coverage.

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[LisaScheers]

recheck