Translate the new-format network tables to buses and links

[nick-gorman]

This PR adds the translator that turns the new-format network tables into PyPSA buses and links.

network.py consumes the new-format network_geography, network_transmission_paths, network_transmission_path_limits, network_expansion_options and network_transmission_path_expansion_costs tables, and produces:

• one bus per modelled geography (REZ buses only when REZs are modelled as discrete nodes),
• existing links — the winter_reference limit becomes p_nom, the other demand conditions are returned per unit of p_nom in a link_timeslice_limits table,
• expansion links, built per investment period from the unified expansion options/costs tables.

Flow paths and REZ connections run through one pipeline — both are just paths.

The module is dark: nothing calls its helpers yet (that wiring lands with the orchestrator PR), so there's no change to production behaviour.

src/ispypsa/translator/
└── network.py        → new-format network tables → PyPSA buses + links (+ per-timeslice link limits)
tests/test_translator/
└── test_network.py   → unit coverage for the buses/links helpers

[codecov]

Codecov Report

❌ Patch coverage is 97.72727% with 2 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
src/ispypsa/translator/network.py	97.72%	1 Missing and 1 partial ⚠️

Files with missing lines	Coverage Δ
src/ispypsa/translator/network.py	97.72% <97.72%> (ø)

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.