Preserve uv source configuration

[erikgaas]

Summary

• preserve existing [tool.uv.sources] values when workspace projects are synchronized or removed
• serialize workspace, index, Git, path, marker, and list-valued source entries without converting them all to workspace = true
• add regression coverage for index-backed PyTorch sources and mixed source types

Root cause

FastWS parsed the existing source table, but rebuilt every entry as { workspace = true }. Running ws-add could therefore turn an index-backed source such as torch = { index = "pytorch" } into a workspace source and break the next uv sync.

Impact

Newly discovered local projects still receive { workspace = true }, while external source configuration created manually or by uv add remains intact.

Validation

• uv run --with pytest pytest fastws/tests -q — 26 passed
• uv sync --dry-run in the workspace — no changes