Add juv stamp for time-based dependency resolution pinning

[manzt]

This PR introduces the juv stamp command, which sets an exclude-newer timestamp for constraining dependency resolution in uv by time. This approach significantly improves the ergonomics of using this feature for little scripts, especially in the absence of traditional lockfiles.

By pinning a specific timestamp, juv stamp ensures that uv resolves dependencies as they existed at that point in time. This means you can stamp your scripts before you share them with others, or when you commit scripts or notebooks to version control systems.

New Command: juv stamp

The juv stamp command pins a timestamp as exclude-newer in the metadata of the target file, effectively locking the dependency resolution context:

juv init foo.ipynb

# Stamp the current time
juv stamp foo.ipynb

# Stamp a specific timestamp
juv stamp foo.ipynb --time "2020-03-05T00:00:00-05:00"

# Stamp using a date (resolves to midnight in the system's local timezone)
juv stamp foo.ipynb --date 2022-01-03

# Stamp a specific Git revision
juv stamp foo.ipynb --rev e20c99

# Stamp the latest commit time
juv stamp foo.ipynb --latest

The juv stamp command also works with regular Python scripts. This feature will land eventually in uv (via a config command, but until then you can use this CLI for your uv scripts with uvx:

uv init --script foo.py
uv add --script foo.py polars anywidget
uvx juv stamp foo.py