ci(audience): route all matrix jobs through set-matrix, harden Windows checkout (SDK-330)

[ImmutableJeffrey]

Summary

• Restores audience PlayMode CI on main. Since SDK-327 was merged, the playmode job has been silently expanding to 0 cells (only the 6 mobile-build cells ran on every PR). Root cause: the cross-product matrix had a unity-keyed include item that, after the conditional exclude removed Unity 2022 on PR runs, spawned an orphan combination missing target/backend/runner; runs-on: ${{ matrix.runner }} then evaluated to empty and GitHub aborted the entire matrix.
• Replaces the cross-product + axis-matching includes with a set-matrix helper job that emits fully-specified JSON matrices for playmode, playmode-linux, and mobile-build, conditioned on github.event_name. Each cell in the JSON carries every key the steps need, so no augmentation step can silently drop keys.
• All three matrix-driven jobs declare needs: set-matrix and consume matrix.include: fromJSON(needs.set-matrix.outputs.<key>). The workflow graph now shows all three matrix jobs hanging off set-matrix in one place; the matrix definitions are also colocated in one step for easier maintenance.
• PR runs strip Unity 2022.3.62f2 cells from playmode (8/12), playmode-linux (4/6), and mobile-build (4/6) via a single jq filter. Schedule and workflow_dispatch get the full sets.
• Hardens the Windows pre-checkout step. The previous Kill-stale step only matched a narrow Unity-family process list and slept 2 seconds before handing off to actions/checkout@v4, which would die with EBUSY on stuck files in examples/audience. The new step adds bee_backend and mono* to the kill list, sleeps 3 seconds, then force-removes the workspace contents in a 6-attempt retry loop so checkout's own cleanup is left with nothing to do.
• SDK-329 (mobile-build trigger if-fix) and the upstream Unity 6 alignment (mobile 6000.4.5f1 → 6000.4.0f1) have since merged into main. This PR rebased on top, so mobile-build's if keeps the explicit (pull_request && !fork) || schedule || workflow_dispatch form and mobile uses 6000.4.0f1 like the other jobs.

Coverage matrix after merge

Job	PR (non-fork)	Schedule / workflow_dispatch
playmode (Win + macOS)	8 cells (IL2CPP + Mono2x × {2021, Unity 6})	12 cells (adds 2022)
playmode-linux	4 cells (IL2CPP + Mono2x × {2021, Unity 6})	6 cells (adds 2022)
mobile-build (Android + iOS)	4 cells (IL2CPP × {2021, Unity 6})	6 cells (adds 2022)
Total	16 cells	24 cells

Mobile is IL2CPP-only by design (Unity mobile production builds don't use Mono2x); that predates SDK-327 and is unchanged.

Linear: SDK-330

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