Remove unused using directives

[anagnorisis2peripeteia]

Part of #38 (dead-code audit).

What

Remove 158 unused using directives across 96 files in DS4WinWPF, flagged by the IDE0005
analyzer during a full-reference build. Pure removal — no code changes.

Why the extra care (WPF makes this non-trivial)

Unused-using detection is unreliable on this project via the obvious route: dotnet format's
IDE0005 pass runs in a Roslyn workspace that doesn't fully resolve the project's package
references (ViGEm / FakerInput / OSC), so it falsely flags live usings as unused (e.g.
Nefarius.ViGEm.Client) and would break the build.

So this PR removes only the usings flagged by IDE0005 during a real, full-reference
dotnet build (EnforceCodeStyleInBuild=true), and then verifies every removal by
compiling — no removal is trusted until the compiler proves it unnecessary.

How verified

All four shipping configurations compile with zero errors (a using needed only by an
#if WIN64 (x86) or #if DEBUG branch would fail one of these):

x64 Release publish : REL_X64_EXIT=0
x86 Release publish : REL_X86_EXIT=0
x64 Debug build     : DBG_X64_EXIT=0
x86 Debug build     : DBG_X86_EXIT=0
DS4WindowsTests     : Passed 11, Failed 1, Total 12

The single test failure (CheckSettingsRead) is a pre-existing en-GB date-locale snapshot
mismatch (Expected 05/12 vs Actual 12/05), identical on untouched upstream/main — not
introduced here.

Authored with Claude (Opus 4.8).

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Review history: https://gist.github.com/81d5826c23e2eb0dfdedaade7421e647