Cleanup: instance-style extension calls, redundant generic args, and redundant casts

[ivandrofly]

Three small, behavior-preserving cleanup commits:

1. Drop redundant <string> type argument from MakeComboBox calls — the generic type is inferred from the source collection, so the explicit <string> was just noise. Touches the DCinema SMPTE, iTunes Timed Text, Timed Text 1.0, IMSC 1.1 property windows, and the Cut Video window.

2. Use instance-style extension method calls — convert static extension-method invocations to instance call syntax: LINQ (Enumerable.FirstOrDefault/Count/ToList), ObservableCollectionExtensions.AddRange, ClipboardExtensions.TryGetTextAsync, UiUtil.RemoveControlFromParent, and SplitToLines.

3. Remove redundant and unnecessary casts — drop casts the compiler already supplies or that are no-ops: redundant numeric casts around Math.Round/division/TimeSpan.FromSeconds (integer-division semantics preserved), pointless (uint)/(int)/(byte) casts on already-correct types, leftover (IEnumerable<T>) upcasts before LINQ instance calls, and superfluous nullable/reference casts on ternary branches.

No behavior change. Full solution builds clean.

🤖 Generated with Claude Code

[niksedk]

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[niksedk]

Reviewed hunk by hunk with focus on the cast removals, since "redundant" casts around arithmetic are where cleanups silently change semantics. All verified behavior-preserving:

• The numeric-division hunks keep their original semantics: every removed (double) sat where a * 100.0 / - 10.0 / / 2.0 literal already forced double arithmetic, or (in AdvancedEffectAudioTextPulse) the cast applied to the result of the truncating long division, which is unchanged.
• The overload-resolution traps check out: TimeCode(double) and Paragraph(string, double, double) are the only matching constructors, so the implicit ulong→double conversions pick the same overloads the casts did.
• SeWindowPosition.Width/Height are int, so the window-centering (int) casts were true no-ops (not double→int truncations).
• nuint→ulong in the mpv stride write is an implicit, value-preserving conversion.
• ClipboardExtensions is Avalonia's own extension class and IClipboard has no instance TryGetTextAsync, so instance syntax binds to the same method.
• The target-typed null ternaries and ?? 0 coalescing hunks are compile-proven (a type mismatch would not build).

One leftover fixed on top: FontNames.FirstOrDefault<string>() kept the redundant generic argument the same commit was removing elsewhere.

Full solution builds clean; all 1,496 tests pass (libse, libuilogic, seconv, UI).

🤖 Generated with Claude Code