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• Cancel superseded Unity test and benchmark runs so stale work cannot starve the self-hosted runner fleet.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

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

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Note

High Risk
Large CI matrix and concurrency changes affect licensed Unity runs, release exports, and runner provisioning; incorrect cleanup or version pins could block merges or ship the wrong editor build.

Overview
This PR adds Unity 6000.6.0f1 as the canonical release/latest editor, drops 6000.3 from CI, and threads the new version through issue templates, unity-versions.json, release export, and unitypackage smoke pins with drift guards.

CI behavior changes: Unity Tests now matrix on Unity version only—each job runs editmode, playmode, and standalone (plus acceptance on dispatch) under one lock per version, with cancel-in-progress concurrency so superseded runs release the fleet faster. Benchmarks collapse to a single 6000.6 playmode leg (no multi-version matrix or version dispatch input). Runner bootstrap splits host prerequisites from a serialized per-version Unity maintenance matrix. Failed-test diagnostics ignore failed suites whose failure site is Child, avoiding duplicate aggregate suite noise.

Product fixes: Seven UI Toolkit progress controls gain Unity 6 [UxmlElement] / [UxmlAttribute] paths while keeping legacy UxmlTraits on 2021/2022; new editmode UXML tests and EditorTestCheck Styles inclusion back that. PooledArrayBufferWriter rejects invalid Advance/EnsureCapacity without corrupting state. Broad exception logging switches from Message to full exception text (tests updated for repair flows).

Docs/policy: Changelog entries for #759–#762; agent guidance names wallstop, Cursor Bugbot, and Copilot bots as inputs that do not require a human pause.

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