Gate FFmpeg library download to Windows x64

[Blackspirits]

Purpose

Independent platform-capability follow-up to #134 ("Pin and verify the FFmpeg shared-library archive").

#134 intentionally supports one reviewed native package: Windows x64. The service already rejected every other platform at runtime, but Settings exposed the action on every Windows process, including native Windows ARM64/x86 where it could only fail.

Current upstream SubtitleEdit/subtitleedit main was rechecked at d6917286e355976da953d0ee22208b4bf9461699; no relevant drift or open upstream PR was found.

Finding fixed

Settings used OperatingSystem.IsWindows() while FfmpegLibsDownloadService.GetUrl() required Windows and process architecture x64. Windows ARM64/x86 therefore saw an action whose only outcome was PlatformNotSupportedException.

Fix

The service now owns one capability contract:

• pure IsDownloadSupported(bool isWindows, Architecture processArchitecture);
• runtime IsDownloadSupportedOnCurrentPlatform();
• GetUrl() uses that capability;
• Settings uses the same capability for IsFfmpegLibsDownloadVisible.

Future platform support therefore changes from one contract rather than two independent conditions.

Regression coverage

The matrix pins:

• Windows x64 -> supported;
• Windows ARM64 -> unsupported;
• Windows x86 -> unsupported;
• non-Windows x64 -> unsupported.

Existing #134 URL/digest tests remain unchanged.

Scope / branch state

• dependency/base PR: Pin and verify the FFmpeg shared-library archive #134
• base HEAD: 88f1405ec15e2b73d9192c0307cb2efc3da65d3e
• HEAD: 40a378520cf3c769c01b2ade05d9fc82f68b2cdc
• 1 commit
• 3 files
• +27 / -2
• current upstream main checked: d6917286e355976da953d0ee22208b4bf9461699
• audit draft only; no merge/promotion intended

AI assistance: ChatGPT was used for platform-capability review, UI/service contract deduplication, upstream-conflict search and architecture-matrix regression design.

Final CI

Authoritative run: SubtitleEdit#274 on 40a378520cf3c769c01b2ade05d9fc82f68b2cdc

• SeConv: 488 passed / 2 skipped / 0 failed
• LibUiLogic: 905 passed / 0 skipped / 0 failed
• LibSE: 2017 passed / 0 skipped / 0 failed
• UI: 5237 passed / 9 skipped / 0 failed
• build: 0 errors / 7 pre-existing warnings
• retry step skipped
• failure-artifact upload skipped

Final adversarial review was performed on this exact HEAD after CI; no blocker found.

[Blackspirits]

Final adversarial review on 40a378520cf3c769c01b2ade05d9fc82f68b2cdc after CI SubtitleEdit#274: no blocker found. The Settings affordance and service URL selection now consume the same Windows-x64 capability contract; ARM64/x86 cannot be offered a guaranteed-failing action, while the service still fails closed if invoked directly. No merge/promotion performed.