Validate negative startIndex in ObservableList.RemoveRange

[VPDPersonal]

Summary

Adds a startIndex < 0 guard to ObservableList<T>.RemoveRange(int startIndex, int count) (Runtime/Observable/ObservableList.cs:171), alongside the existing count < 0 check and before the lock (SyncRoot). Without it, a negative startIndex would fall through to _list[startIndex + i] inside the lock and surface as IndexOutOfRangeException (or ArgumentOutOfRangeException from the underlying list) — depending on count, possibly only after partially observing the wrong slice.

With the guard, callers now get a deterministic ArgumentOutOfRangeException(nameof(startIndex)) up front, mirroring the BCL List<T>.RemoveRange contract and the sibling count check that already exists one line below.

Notes for review

• Single-line addition, no behavior change for valid input.
• Exception is thrown before the lock is taken — same ordering as the existing count guard, so there is no risk of holding SyncRoot while throwing.
• No tests added: the precondition is symmetric with count < 0, which is already covered by the existing RemoveRange negative-input tests; the same shape applies here.

Test plan

• Aspid.Collections.Observable.Tests — full EditMode suite still passes.
• Manual: new ObservableList<int> { 1, 2, 3 }.RemoveRange(-1, 0) throws ArgumentOutOfRangeException with ParamName = "startIndex".

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Generated by Claude Code