--opt= silently collapses to a bare flag and consumes the following token (short -o= behaves differently)

## Description

`ParsedArgument.init(_:)` (`Sources/ArgumentParser/Parsing/SplitArguments.swift`, ~lines 21–31) collapses an empty value after `=` into a bare flag:

```swift
let indexOfEqualSign = str.firstIndex(of: "=") ?? str.endIndex
let (baseName, value) = (str[..

Start in Sources/ArgumentParser/Parsing/SplitArguments.swift, especially ParsedArgument.init(_:) around lines 21–31 and the short-option handling around lines 743–750. Trace parseIndividualArg and popNextElementIfValue(after:), then add regression coverage for ["--out=", "file.txt"] and ["-o=", "file.txt"]. Done means both forms preserve an explicitly empty value and leave file.txt positional.

Written by the indexing model from the issue text.