Better monorepo venv support

Stations now find the right Python environment every time, including monorepo deployments where the venv lives next to a sub-package.

• Improved monorepo support: venv is resolved deterministically at the package directory, no more walk-up search
• Fixed run failures on stations where divergent venv resolvers picked the wrong interpreter
• Added regression tests covering monorepo, single-package, and local-path runs