fix(parse): honor separator after automatic args

[jdx]

Replaces #1152, which GitHub automatically marked merged when dependent branch refs temporarily matched during a restack. This is the same layer on the corrected stack.\n\n_This pull request was generated by Codex._

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Note

Medium Risk
Changes argv -- semantics in both usage-argv and usage-lib, which can rebind nested passthrough command lines. Coverage is added, but this is core parser behavior.

Overview
Fixes nested passthrough like mise run wrapper -- command ...: an automatic argument used to turn a later explicit -- into data, so a double_dash="required" field never unlocked and the wrapper received -- as its executable.

The first explicit separator now stays syntax after automatic flag stopping. Only a separator that has already been consumed makes later -- tokens ordinary values. The same rule is applied in usage-argv and usage-lib, and the mise tasks run gate plus a new Split conformance case assert both the typed parse and the emitted spec.

^{Reviewed by Cursor Bugbot for commit afb08bf. Bugbot is set up for automated code reviews on this repo. Configure here.}

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