feat: run conda-script files via pixi run --experimental --script

[Hofer-Julian]

pixi run --experimental --script FILE runs conda-script files (#3751) end to end.

• --experimental gates the new path and has no effect for PEP 723 scripts, so #!/usr/bin/env -S pixi run --experimental --script works for both block kinds; a conda-script file without the flag errors with a hint naming it
• The block synthesizes a workspace through the existing script machinery: [dependencies] fills the default feature and [tool.pixi.*] becomes a second feature of the default environment, so both spec sets reach the solver as a union
• The environment resolves for the machine, an adjacent lock file wins when present, and pixi lock --script FILE writes FILE.pixi.lock for conda-script files too
• The entrypoint runs through the mini-shell with ${SCRIPT} and ${CACHE} defined and the working directory left at the invocation directory
• A malformed pseudo-block in a .py file falls back to the PEP 723 path, so files that ran before keep running; initializing a file that already carries a conda-script block is refused
• Integration tests cover the gate, the routing, argument forwarding, ${CACHE} persistence, when dependencies and locking

Considerations before merging

• pixi lock --script accepts conda-script files without --experimental, since the flag only exists on run; locking a file with source dependencies fetches and builds them, like a PEP 723 script with the pixi-build preview does
• Leading-hyphen arguments after the script path need a -- separator, the same as for PEP 723 scripts; a shebang line cannot insert one
• The entrypoint's child processes get no kill-signal drop guard yet, unlike task execution