Configure pixi for cross-platform environments

[jdbcode]

Summary

Enable pixi setup to work across common developer platforms and tighten pixi-related repo config.

Changes

• Add cross-platform pixi workspace targets: linux-64, osx-64, osx-arm64, win-64
• Keep editable local package install via pixi pypi dependency (xee = { path = ".", editable = true })
• Align examples pixi environment to include both examples and dataflow features
• Improve pixi lock handling in git attributes (merge=binary, -diff)
• Refine .gitignore handling for .pixi/ while keeping .pixi/config.toml tracked

Validation

• pixi project channel list succeeds and reports expected environments on conda-forge.

Notes

• This PR intentionally avoids introducing new pixi package pins to minimize behavior changes; it focuses on portability and setup hygiene.