Generate XKFX overrides as a separate, non-destructive include file rather than regenerating XKFX.inc

The backcast pipeline only computes Observed Max Flow ([[0005]]) for borders with a clean 1:1 (or aggregatable) mapping between a Balmorel region and an ENTSO-E bidding zone, for a single target year. Every other entry in XKFX.inc — inter-DE4-* internal links, and any border ENTSO-E has no usable data for — must keep its current value untouched.

Rather than parsing and regenerating the entire XKFX.inc (its existing symmetric base TABLE plus hand-tuned asymmetric overrides), the pipeline writes a separate XKFX_ENTSOE.inc containing only XKFX('<year>','A','B')=value; assignment lines for the borders it computed — the same style already used for the hand-written NO1/NO2/NO5 corrections. This file is pulled in via Balmorel’s standard scenario-fallback $INCLUDE pattern (checking <scenario>/data/ first, falling back to base/data/), which the script appends to the end of the base XKFX.inc itself (idempotently — it checks whether the block is already present before appending, so re-running the command doesn’t duplicate it):

$if     EXIST '../data/XKFX_ENTSOE.inc' $INCLUDE         '../data/XKFX_ENTSOE.inc';
$if not EXIST '../data/XKFX_ENTSOE.inc' $INCLUDE '../../base/data/XKFX_ENTSOE.inc';

Because GAMS assignments execute in file order, anything the override file doesn’t mention is left exactly as the base table set it. The script only ever appends this fixed two-line block to XKFX.inc — it never parses or rewrites the base table — so it cannot corrupt or silently drop a link it doesn’t understand, which a full-file regeneration could.

Two aggregation rules apply when writing the override file, needed because Balmorel and ENTSO-E don’t always share the same regional resolution:

Many ENTSO-E zones → one Balmorel region (e.g. Italian sub-zones → IT): sum the observed flows.
One ENTSO-E zone → many Balmorel regions (e.g. DE_LU → DE4-S/DE4-W): split the observed flow evenly across the matching Balmorel sub-links.

The set of borders to fetch ENTSO-E data for is derived from the non-zero entries already present in the target XKFX.inc, translated to ENTSO-E zone codes and de-duplicated, rather than a hand-maintained list — this keeps the fetch scope automatically in sync with whatever interconnections the Balmorel model actually defines, and pairs that translate to the same zone on both sides (the internal DE4-* links) are naturally excluded. fetch_annual_transmission_data already skips a pair if its CSV was already downloaded. Any pair that still fails to fetch is surfaced as a warning at the end of the run, rather than silently dropped, so the user can decide whether to fix a zone-code mapping or proceed with the borders that were available.

Cleaning up XKFX.inc’s ad-hoc hand-formatting is a separate, later concern and out of scope for this pipeline.