feat(weather): per-zone weather for the ENTSO-E four-zone pipeline

[bartzbeielstein]

Summary

When ConfigMulti.per_zone_weather=True, each of the four German TSO control zones (load_50hertz/load_amprion/load_tennet/load_transnetbw) is driven by weather from its own region instead of one shared national index. Calendar, holiday and event-window features stay shared (national); only weather becomes regional, under the same un-prefixed column names so nothing downstream changes. Opt-in, default OFF → byte-identical to the prior shared-weather baseline.

This closes the gap the four-zone bottom-up KB entry left open ("reconciliation/stacking deferred").

Mechanism (ADR-approved)

The existing single global weather build stays as the shared schema/baseline; build_exogenous_features additionally fetches one weather frame per zone (reusing the multi-city get_weather_features path with each zone's city list) into self.zone_weather_aligned. At the single per-target seam (get_target_data, new keyword-only zone_weather) the zone's weather columns overwrite the shared values in-place — column order/shape preserved. The per-zone frame keeps its native [start, cov_end] index so it spans the forecast horizon (regression-tested). On all-zones-success the shared schema is seeded from the first zone, so per-zone weather survives a global fetch that failed under on_weather_failure="skip".

Changes

• weather/locations.py: registry 13→15 (Mannheim, Karlsruhe for TransnetBW); frozen GERMAN_TSO_ZONE_CITIES partition (provenance: SMARD control-area map + TSO wiki; Hamburg∈50Hertz, Bremen∈TenneT); locations_for_zone resolver (fail-safe ValueError on unknown zone). Pre-existing Sphinx roles swept.
• configurator/: per_zone_weather + zone_weather_locations fields; validate_config guards reject combination with use_population_weighted_weather (mutual exclusivity), use_exogenous_features=False, and poly_features_degree>=2.
• Fail-safe: under "skip" any zone fetch failure degrades the whole pipeline to no-weather (never substitutes the global index); under "raise" aborts naming the zone.
• tasks/task_safe_zone_load_demo.py: --per_zone_weather flag (default off, offline-safe).
• 36 network-free tests incl. mapping/partition, config guards, the seam overwrite, forecast-horizon coverage, skip-degradation, and non-zone-target fail-safe. quartodoc reference + freeze regenerated.

Verification (all green locally)

• uv run pytest tests/ -q → 2635 passed, 1 skipped
• ruff / black / isort / reuse lint → clean
• quartodoc build + interlinks → API reference in sync
• quarto render --no-cache → all 191 pages, exit 0

Authoring: tbb-skills4agents agents (software-architect → code-writer → code-reviewer) with orchestrator review that caught & regression-tested a forecast-horizon truncation bug.

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