Augmentation modelling decisions for CNSW-SNW (and any future split parallel paths)

[nick-gorman]

The IASR splits the CNSW-SNW corridor into two parallel paths
(CNSW-SNW_NTH, CNSW-SNW_STH) but augments capacity through a single
un-suffixed flow_path_augmentation_options_CNSW-SNW table. Each option
in that table carries a Development path column with values like
"Northern side of SNW", "Southern side of SNW", "Bayswater to Newcastle",
"Bannaby to Dapto", and "Northern side of Sydney" — the source data does
distinguish where each option lands, but the mapping to existing path_ids
(or to genuinely new corridors) requires interpretation.

Decision (templater branch new-format-network-tables):
The augmentation is modelled as a single new parallel path. The templater
adds an un-suffixed CNSW-SNW row to network_transmission_paths (CNSW
→ SNW, AC) with explicit zero existing capacity in
network_transmission_path_limits (6 rows: 2 directions x 3 timeslices,
all 0 MW). The selected option's Development path value is dropped.

Rationale: PyPSA represents augmentation as a new Link regardless, so the
three options (NTH / STH / new corridor) are operationally equivalent
unless a custom constraint references the parallel paths with different
coefficients.

Limitations:

• A custom constraint that distinguishes CNSW-SNW_NTH, CNSW-SNW_STH and
  the expansion (e.g. different coefficients per parallel path) cannot be
  represented exactly — the expansion isn't tied to either existing
  parallel path.
• Augmentation options whose Development path is "Bayswater to Newcastle"
  or "Bannaby to Dapto" describe genuinely new corridors with their own
  endpoints; modelling them all as CNSW-SNW (CNSW → SNW) is a topology
  approximation.
• Generalises to any future workbook version where a flow path splits into
  parallel suffixed paths but augments through the un-suffixed key. The
  existing logic in network_expansion._new_parallel_path_rows will pick
  these up automatically and apply the same simplification.

Resolution path if/when needed:
Build a Development path → path_id mapping (NTH/STH for the explicit
side cases; distinct new-corridor path_ids for the others) inside the
augmentation templater. Most options would attach to existing parallel
paths; only genuinely new corridors would create new topology rows.