fix: make locale-independence real, not assumed — parsing, casing, comparison

Follow-up to the retail-text fix. "Green under sv-SE" is not the same as "runs
on any locale", so this establishes the latter by running the suite under
cultures chosen to break different things, and fixing what they broke.

ar-SA found a genuine defect the Swedish runner cannot see: the resolution
parser read "1920x-1" through the ambient culture, and ar-SA's negative sign is
not ASCII '-', so the parse failed and the height silently became 0 instead of
-1. Both copies of that parser (App settings targets and the UI settings store)
now parse invariantly.

Audited every remaining culture-sensitive operation in src/ rather than fixing
only what a test happened to catch:

- Numeric Parse/TryParse with no IFormatProvider: 11 sites, all reading
  MACHINE-readable input — env vars (ACDREAM_LIGHT_DEBUG, ACDREAM_NET_DROP_*,
  streaming/quality knobs), CLI arguments, "1920x1080" settings keys, a chat
  command's price argument, and the launcher's bake thread count, which is
  handed straight to a child process command line. All pinned to
  InvariantCulture.
- ToUpper()/ToLower() with no culture: none. The Turkish-I class was already
  clean, and tr-TR confirms it.
- StartsWith/EndsWith/IndexOf(string) with no StringComparison: one —
  ChatInputParser's "@" prefix test, which is a culture-sensitive comparison
  for a single ASCII character. Now the ordinal char overload.

Verified: 13,958 tests pass identically under the machine default, sv-SE,
tr-TR, ar-SA, and de-DE. (The two launcher test assemblies are excluded from
this run only because a running acdream-launcher.exe holds its own binary; the
one launcher change here is the thread-count parse.)

Dates remain on the current culture by intent, unchanged from the previous
commit.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

src/AcDream.Launcher/ViewModels/FirstRunInstallerViewModel.cs

@@ -1,3 +1,4 @@
+using System.Globalization;
 using System.ComponentModel;
 using AcDream.Launcher.Core.Installation;
 using AcDream.Launcher.Core.Launching;
@@ -90,7 +91,9 @@ public sealed class FirstRunInstallerViewModel : ObservableObject, IDisposable
     }
 
     public bool IsThreadCountValid =>
-        int.TryParse(ThreadsText, out int threads) && threads > 0;
+        int.TryParse(
+            ThreadsText, NumberStyles.Integer, CultureInfo.InvariantCulture, out int threads)
+        && threads > 0;
 
     public string ThreadCountValidation => IsThreadCountValid
         ? "Worker count is valid."
@@ -324,7 +327,12 @@ public sealed class FirstRunInstallerViewModel : ObservableObject, IDisposable
 
     private async Task StartAsync()
     {
-        if (!int.TryParse(ThreadsText, out int threads) || threads <= 0)
+        if (!int.TryParse(
+                ThreadsText,
+                NumberStyles.Integer,
+                CultureInfo.InvariantCulture,
+                out int threads)
+            || threads <= 0)
         {
             return;
         }