Shkarko usage 5.0.0

A parser-level fix that makes double_dash="required" actually behave as declared drives the major bump: values before -- are now rejected, and values after -- are routed past greedy variadics to the arg that was waiting for them. The release also fixes a cluster of long-standing Windows problems — usage bash losing every usage_* variable under WSL, run= scripts being handed to cmd /c, and completion guards being fooled by a shell function named usage — and lets generate markdown write to stdout like the other generators.
Added
- Override the shell binary with USAGE_SHELL_<SHELL> (#767 by @JamBalaya56562). Point usage bash, usage zsh, usage fish, and usage powershell at a specific interpreter — mainly so Windows users can escape the WSL bash.exe that Win32's search order picks up ahead of $PATH:
set USAGE_SHELL_BASH=C:\Program Files\Git\usr\bin\bash.exe
usage bash C:/work/mycli
The variable is keyed by the program (so powershell's override is USAGE_SHELL_PWSH). Unset, empty, or whitespace-only falls back to the default. Spawn failures now name the program that was tried and the variable it came from, and on Windows a bash exit 127 against a drive-letter path prints a hint pointing at this override.
- generate markdown writes to stdout (#766 by @JamBalaya56562). --out-file is now optional and defaults to stdout, matching manpage, fig, json, and completion. --out-file - also means stdout on markdown, manpage, and fig, mirroring the -f - input convention. The writing to … progress line moved to stderr on markdown, manpage, fig, and sdk, so it no longer ends up inside the generated document. --out-dir now requires --multi.

Fixed
- double_dash="required" is now enforced on both sides (#762 by @JamBalaya56562). The parser previously ignored SpecDoubleDashChoices::Required entirely — a word offered to such an arg without -- was accepted anyway, and a required arg sitting behind a greedy variadic was unreachable even with a separator. Now offering a value before -- is reported as ArgRequiresDoubleDash (once per variadic, not once per word), and an explicit -- routes the positional cursor onto the arg that required it, past earlier args. Completion learns about -- too: while an arg is locked behind a separator, -- itself is offered rather than values the parser would reject.
- Windows: usage_* variables reach WSL bash (#764 by @JamBalaya56562). On Windows the bash picked up from the system directory is WSL's launcher, and WSL only forwards a Win32 variable when WSLENV names it — so scripts saw every usage_* value unset. Both shell and exec now append the parsed argument names to WSLENV (bare, no /p or /l flags), preserving any entries the user had already configured.
- Windows: run= scripts use sh when available (#765 by @JamBalaya56562). complete run= already used sh -c everywhere, but mount run= used cmd /c on Windows, so the same POSIX one-liner behaved differently depending on which KDL node it lived in — and shebang scripts silently exited 0 with empty output. Both call sites now share one implementation: sh -c first, falling back to cmd /c only if sh is not found. Non-UTF-8 output from either shell is now reported as an error instead of panicking.
- Bash/fish completion guard ignores shell functions (#760 by @JamBalaya56562). The generated completion opens with a guard that bails out when the usage CLI is not installed, but type -p returns exit 0 for a shell function, so any environment defining a usage function (e.g. oh-my-bash) passed the guard and then failed further down with an unrelated error. Switched to type -P in both bash guards and the fish equivalent; zsh's type -p already forces a $PATH search and is unchanged.

Breaking Changes
- double_dash="required" positional args now reject values before -- (#762). Specs where such an arg previously happened to work without a separator will now error. In examples/mise.usage.kdl, post--- values also move from the preceding greedy variadic to the arg that declared the separator (e.g. from TASK_ARGS to TASK_ARGS_LAST, from TOOL@VERSION to COMMAND under exec), which changes which usage_* variable a consumer reads. Spec authors who want the old permissiveness can drop back to double_dash="optional" (the default).
- UsageErr and ParseOutput gained fields. UsageErr has a new ArgRequiresDoubleDash variant, and ParseOutput gained next_arg and double_dash_seen. Library consumers matching these types exhaustively will need to update.

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