Fix Unity 6000.6 progress controls and issue debt

[Eli Pinkerton (wallstop)]

DISCLOSURE: LLM-GENERATED TEXT

Why:

Unity 6000.6 no longer compiles the legacy UI Toolkit progress-control factories, two audited reliability issues could corrupt pooled writer state or discard exception diagnostics, and the Unity CI/reviewer automation needed current repository policy.

What:

• Use generated UXML elements and attributes for all seven progress controls on Unity 6 while retaining the legacy factory path on Unity 2021.3 and 2022.3.
• Preserve Unity 6000.5 coverage, add released 6000.6.0f1, remove obsolete 6000.3, and run edit/play/standalone sequentially inside one job per supported version.
• Run benchmarks once on the latest editor instead of a four-cell matrix, preserving benchmark selection and samples.
• Cancel superseded Unity test and benchmark runs so stale work cannot starve the self-hosted runner fleet.
• Target release export and unitypackage smoke with the latest supported editor, 6000.6.0f1.
• Add checksum-verified Windows runner installation with an AdminPrepare → UserInstall → AdminConfigure handoff, and provision every requested Unity build target through the Full profile.
• Reject invalid pooled writer advances, negative hints, and capacity overflow without mutating writer state.
• Preserve exception types, inner exceptions, and stack traces throughout runtime and editor diagnostic logs.
• Always act on review input from wallstop, Cursor Bugbot, and GitHub Copilot's review/coding agents, with durable linter and self-test coverage.
• Add UXML import/default-size coverage, writer boundary tests, documentation, changelog entries, and CI contract tests.

Fixes #759
Fixes #760
Fixes #762
Fixes #767

Type of Change

• Bug fix (non-breaking change that fixes an issue)
• New feature (non-breaking change that adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update
• Refactor (code change that neither fixes a bug nor adds a feature)

Checklist

• I have added tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have updated the CHANGELOG
• My changes do not introduce breaking changes, or breaking changes are documented

Validation

• npm run validate:tests:fast — 87/87 contract groups passed
• npm run validate:content — passed
• npm run lint:spelling — passed
• Editor test typecheck — zero warnings and errors
• CSharpier, actionlint, YAML, line-ending, and staged-file preflight checks — passed
• Unity 6000.4.6f1 generated-UXML compilation — zero warnings and errors
• GitHub Actions checks for the current head are in progress

---

> [!NOTE]
> High Risk
> Changes licensed Unity CI topology (cancel-in-progress, per-version sequential modes, release editor pin) and release export version; incorrect cleanup or version drift could block releases or starve runners.
>
> Overview
> Unity 6000.6 becomes the release and benchmark editor: .github/unity-versions.json drops 6000.3.16f1, adds 6000.6.0f1, and release/smoke/export pins move from 2022.3.45f1 to 6000.6.0f1. Issue templates list Unity 6.6.
>
> Seven UI Toolkit progress controls gain Unity 6 [UxmlElement] / [UxmlAttribute] registration while keeping legacy UxmlTraits on 2021.3/2022.3, plus new editmode UXML import tests and Styles in the editor test typecheck project.
>
> Runtime fixes: PooledArrayBufferWriter rejects invalid Advance/EnsureCapacity without mutating state; widespread logging now prints full exceptions instead of .Message only.
>
> CI behavior: Unity Tests run one job per editor version that sequences editmode, playmode, and standalone (matrix axis removed); concurrency cancels superseded runs on the same PR/ref. Benchmarks collapse to a single 6000.6.0f1 playmode leg with a drift guard. Runner bootstrap splits host prerequisites from a serialized per-version Unity maintain matrix. verify-unity-results skips failed suites with @site='Child' when reporting suite-level failures.
>
> Policy/docs: LLM github-ops pause rules explicitly cover wallstop, Cursor Bugbot, and Copilot agents; CHANGELOG entries for #759, #760, #762.
>
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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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