Add network_transmission_paths templater

[nick-gorman]

Summary

• Add topology-only network_transmission_paths table under the feature flag, producing path_id, geo_from, geo_to, carrier from IASR flow path and REZ data
• Parallel flow paths preserved via suffix convention (e.g. NNSW-SQ_Terranora, CNSW-SNW_NTH)
• HVDC carriers identified via explicit path ID set
• Update path_id schema description to document the suffix convention
• Pipeline still only works through the templating step

Test plan

• Happy path with AC paths, parallel paths, all three HVDC paths, and REZ connections
• Empty flow paths with populated REZ
• Empty REZ with populated flow paths
• Both inputs empty

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

Files with missing lines	Coverage Δ
src/ispypsa/iasr_table_caching/local_cache.py	68.96% <ø> (ø)
src/ispypsa/templater/create_template.py	90.16% <100.00%> (+0.33%)	⬆️
src/ispypsa/templater/transmission_paths.py	100.00% <100.00%> (ø)

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