Bring network and resource schemas up to the newer schema format

[nick-gorman]

The costs, generators and storage schemas were restructured in #93 with sectioned descriptions (Source tables / Source notes / If absent), per-column absence semantics and numeric value constraints, but the earlier network_ and resource_ schemas kept the old flat style. This aligns them so all schemas share one format ahead of the validation layer landing. Stacked on #109.

src/ispypsa/validation/schemas/
├── network_geography.yaml                        ← source tables; subregion_id absence semantics
├── network_transmission_paths.yaml               ← source tables; parallel-corridor source note
├── network_transmission_path_limits.yaml         ← blank capacity/timeslice meanings; gte on capacity
├── network_expansion_options.yaml                ← least-cost selection source note; gte on expansion
├── network_transmission_path_expansion_costs.yaml ← integer → int; gte on cost
└── resource_limits.yaml                          ← source table; hard-cap penalty semantics

Two choices a reviewer might query:

• No nan_fill anywhere. In these tables a blank cell carries meaning (no physical limit, hard cap, static timeslice) rather than standing in for a default value.
• resource_limits source table is inferred. It has no new-format templater yet, so initial_build_limits comes from the legacy REZ build-limits templater and may need revisiting when the real one lands.

The gte: 0.0 constraints are new spec, not just reformatting — nothing enforces them today, but they become real rules once validation lands.

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[EllieKallmier]

Minor note I think you pointed out earlier - we could just use "Year" rather than "Financial year" in these descriptions to be a little freer (unless is distinct here?)

[nick-gorman]

Yes! nice catch!