Trim unused dependencies: image AVIF encoder, bindgen, postage, windows 0.61

[chris-addison]

Removes dependencies that the Unity Hub build compiles but never uses. Measured on the Hub's own build graph (cargo tree -p hub-gpui, normal and build deps), this takes about 45 crates out of every platform's build, most of them rav1e, one of the slowest crates in the tree.

Changes

• image: default features off. Only the formats gpui's ImageFormat maps are enabled (bmp, gif, ico, jpeg, png, pnm, tiff, webp), plus rayon. The default set brought in avif, which is an AVIF encoder (ravif/rav1e, about 28 crates) that nothing calls; decoding AVIF needs avif-native, which was never on. It also brought in the exr/dds/hdr/qoi/tga/ff decoders, which no ImageFormat variant reaches. Behavior change: image can no longer guess-decode those formats.
• bindgen: 10 crates off Windows and Linux. gpui's macOS build-dependency on it is removed: build.rs never used it, and cargo-shear already listed it as ignored. gpui_media's is now limited to macOS, the only target where its build.rs generates anything.
• postage: now test-only. It's an optional dependency behind test-support, plus a dev-dependency. app/test_context.rs is the only user, and that module only compiles with test-support. This was the main thing pulling in thiserror 1.
• windows 0.61 alias removed. windows-capture goes from 1.5 to 2.0.1, which moves it to windows 0.62, so the windows-061 alias can go. gpui, gpui_wgpu and gpui_windows now use the workspace windows crate, which drops the whole windows-* 0.61 family (8 crates) from Windows builds. Two API changes in 2.0: Frame::timestamp() now returns a Result, propagated through the handler's existing anyhow error, and as_raw_texture() is no longer unsafe.

Verification

• macOS: cargo clippy -p gpui-ce -p gpui_ce_platform -p gpui_ce_media -p gpui_ce_wgpu --all-targets -- -D warnings passes.
• Windows, cross-checked from macOS: GPUI_RENDER_ALLOW_MISSING_DXBC=1 cargo check --target x86_64-pc-windows-gnu -p gpui_ce_windows -p gpui_ce_wgpu -p gpui_ce_platform -p gpui-ce --features gpui-ce/screen-capture passes with RUSTFLAGS=-D warnings. The msvc target needs a Windows resource compiler for embed-resource, so I used gnu. Nothing has run on real Windows yet; this PR's CI is the first real Windows build.
• Two failures I found were already there on 0a96e0a0 without these changes:
  • Windows clippy reports redundant_clone at gpui_windows/src/directx_renderer.rs:611.
  • --all-features fails to compile bench_context.rs and debug_overlay.rs.
• cargo fmt --all -- --check is clean.

Out of scope, as separate follow-ups:

• block/objc → block2/objc2: block 0.1.6 triggers a warning that a future Rust version will reject it, and the move touches about 275 msg_send! sites.
• wgpu/naga 30 and accesskit 0.27/0.35.
• naga-old.

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