Pin the Playport FEX and DXMT runtime ports

Pins Madeira to the three Playport runtime ports:

• FEX x87 exact integer preservation: https://github.com/willfaust/FEX/pull/35
• DXMT process-local video texture sharing: https://github.com/willfaust/dxmt/pull/50
• DXMT colour typeless texture compression: https://github.com/willfaust/dxmt/pull/51

Review the component PRs first. The DXMT pin merges both branches and retains both regression targets. It contains both component commits as ancestors. No BCD changes are included.

Validation: FEX IR generation and both changed C++ units compile. The three changed DXMT production units and both new regression sources cross-compile together for ARM64EC. The pin commits are available on the respective repository branches.

Draft: source pins do not replace the checked-in runtime binaries. Build through the existing runtime CI route and run assembly, GPU and physical-device tests before release. This PR has no dependency on the upcoming BCD port or the four app PRs.