[Draft] fix Metal 4 Sodium arena lifetime and terrain generation

[21Z121Z1]

Scope

This draft publishes the local Metal 4 arena-lifetime and terrain-generation WIP as an independent review line. It is intentionally based on the canonical Iris integration line and is not an automatic merge target.

• Head: `82bf939`
• Base: `integration/iris-metal-next@a6abdf8`
• Local source preserved at `wip/local/metal4-arena-lifetime`; the original dirty worktree remains archived and untouched.
• Files: 23 source, test, resource, and audit-document files.
• Proposed branch: `fix/metal4-arena-lifetime`

Changes

• Add the Metal 4/Sodium arena-lifetime protection path.
• Add terrain mesh generation ownership and Sodium mixin registration.
• Add upload/copy/mipmap deduplication and render-pass binding updates.
• Add focused terrain-generation and mixin-registration tests.

Verification

• PASS: `./gradlew test --tests 'com.metallum.client.terrain.TerrainMeshGenerationTest' --tests 'com.metallum.mixin.MetallumMixinRegistrationTest' --no-daemon`
• PASS: Java tests, native compilation, resource processing, and test execution in the publish worktree.
• Warnings only: Swift availability and existing unused-variable warnings.

Still open

• Same-world runtime and visual parity have not been closed.
• Paired performance/frame-time evidence has not been measured.
• Iris integration conflict/interaction review is still required.
• No automatic merge is requested; review and validation must happen before landing.

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