Remove unreferenced private members

[anagnorisis2peripeteia]

Part of #38 (dead-code audit).

What

Remove 53 unreferenced private members flagged by the Roslyn analyzers (IDE0051 "unused
private member" plus the CS0169/CS0414 field diagnostics) — members with zero references
anywhere in the compilation:

• DS4StateExposed: 27 implicitly-private pass-through property getters
  (bool Square { get { return _state.Square; } } …) — an abandoned partial "expose state"
  surface. The public API (Motion, etc.) is untouched.
• Device classes (JoyConDevice, SwitchProDevice): dead named constants and an empty
  RunBluetoothSetup() stub.
• Orphaned private helpers: Mapping.TValue / IfAxisIsNotModified / one dead EndMacro
  overload, ScpUtil.applyRatio (Color overload) / ClampInt / GetGyroOutModeString / …Type,
  Mouse.AddEmptyTrackballEntry, CurrentOutDeviceViewModel.DetermineDesired*,
  RecordBox.RevertListItemTemplate, and a few dead fields/constants (tempBusThread,
  macroEndIndex, prevBattery, DBT_DEVNODES_CHANGED, …).

Pure deletion — no behavior change.

Why

These are demonstrably dead: private scope makes the whole reference set the compilation
itself, and the analyzers report zero references. Removing them trims analyzer noise and dead
weight from the tree.

How verified

• Diagnostics collected at build time (EnforceCodeStyleInBuild=true) over the full project.
• Blast radius: x64 and x86 dotnet publish both succeed with zero compile errors — a
  wrongly-removed member would fail to compile. Each removal was additionally cross-checked
  against call sites (a name collision with a live Mapping.ClampInt in a different class was
  caught, and that method was kept).
• Tests: DS4WindowsTests 11/12 pass — identical to untouched upstream/main. The single
  failure (CheckSettingsRead) is a pre-existing date-locale snapshot mismatch (Expected
  05/12 vs Actual 12/05), verified to fail identically on the unmodified baseline.

Build/test evidence (captured, branch f321b56, .NET 8 SDK 10.0.301, Windows)

RESTORE_EXIT=0
PUBLISH_X64_EXIT=0        # x64 dotnet publish — 0 compile errors
PUBLISH_X86_EXIT=0        # x86 dotnet publish — 0 compile errors
DS4WindowsTests (x64): Passed: 11, Failed: 1, Total: 12

The single test failure (CheckSettingsRead) fails identically on untouched
upstream/main (ba4bdcb): Expected:<05/12/2023 00:24:15> vs Actual:<12/05/2023 00:24:15> — a pre-existing en-GB date-locale snapshot mismatch, not introduced here.
For a pure-deletion change, "behavior unchanged" is the deliverable, and the clean
dual-platform compile + baseline-identical test result is the proof of it.

Scope

Deliberately limited to whole-declaration removals of unreferenced private members. Held
back for separate follow-ups: fields that are assigned-but-never-read (those need call-site
edits), and two members whose "unused" status looks like a latent wiring bug rather than cruft
(a commented-out auto-profile event subscription and an uncalled exclusive-mode failure
warning) — those are being raised separately rather than silently deleted.

Authored with Claude (Opus 4.8).