Fix legacy Green/Blue lightbar channels parsing the Red element's value

[anagnorisis2peripeteia]

In ProfileDTO.PostProcessXml, the legacy per-channel lightbar colour path parses the wrong source strings: the green and blue channels both read RedColorString instead of GreenColorString / BlueColorString. A profile that stores its colour as the old separate <Red>/<Green>/<Blue> elements (rather than the combined <Color>) loads with all three channels set to the red value, so the lightbar shows the wrong colour.

The fix: parse each channel from its own element (two-line change), plus a regression test that deserializes a legacy per-channel profile (Red=10, Green=20, Blue=30) and asserts the mapped DS4Color channels individually.

Before/after:

• Before: the new test fails — Assert.AreEqual failed. Expected:<20>. Actual:<10> (green channel received red's value).
• After: it passes; the full suite (with the repo's documented CI exclusions) shows no new failures.

Bug 2 of #46 — found while mutation-testing the profile-serialization code (#37): the surviving mutants pointed at this block being unasserted, and reading it turned up the copy-paste slip.

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_{Local review history: https://gist.github.com/anagnorisis2peripeteia/b97ed542427a5e3fe0184c8074873a24}

Proof manifest

Before / After (same command, same machine)

dotnet test DS4WindowsTests\DS4WindowsTests.csproj -c Release -p:Platform=x64 --filter "Name=CheckLegacyPerChannelColorRead"

Run	Result
Base (ba4bdcb)	❌ Assert.AreEqual failed. Expected:<20>. Actual:<10>. — green channel received the Red element's value
This branch (aef33d4)	✅ passes; full suite shows no new failures; x64/x86 publishes clean

Structured before/after manifest for the red-green evidence already in this description (regression test Expected:<20> Actual:<10> on base; passes on this branch; no new failures in the full suite).

Manifest + raw run outputs: https://gist.github.com/anagnorisis2peripeteia/5f618fc106fac64e4baa18fc28636a8e