add Biochar as carbon removal technology

[BertoGBG]

Closes # (if applicable).

Changes proposed in this Pull Request

This PR introduces Biochar as a Carbon Dioxide Removal (CDR) technology, based on #2141.
Biochar is a land-based CDR technology and its technical potential is related to land availability. As with enhanced rock weathering, the land available is calculated from the CORINE Land Cover dataset using the shared build_available_land.py script, which aggregates a per-bus land-eligibility availability matrix into eligible land area pernetwork node (in km²). That availability matrix is itself produced by a new determine_carbon_dioxide_removal_availability_matrix rule, which reuses the existing scripts/determine_availability_matrix.py unchanged (the same script already used for onwind/offwind/solar). Eligibility is governed by CORINE codes for non-irrigated arable land (potential biochar feedstock land).

#NOTE: biochar's land-eligibility settings (corine, natura, cutout, excluder_resolution) live under renewable: rather than a CDR-specific namespace, for the same reason discussed in the enhanced rock weathering CDR PR: build_available_land.py/determine_availability_matrix.py reuse the same atlite ExclusionContainer machinery and config shape already used for onwind/offwind/solar, read generically via config_provider("renewable", w.technology,
...). It's a pragmatic reuse of existing land-eligibility infrastructure, not a claim that biochar is a power-generation technology. Open to moving it to a separate namespace if preferred — worth deciding consistently across all land-based CDR technologies (rock weathering, biochar, and afforestation to follow) rather than per-PR.

The actual implementation of the CDR technology is part of scripts/prepare_sector_network.py (add_biochar()), which adds biochar as Bus/Store/Link components sharing a single co2 biochar carrier, the Link converts solid biomass
and electricity into squestered biochar-carbon (drawn from the atmosphere), and optionally exports pyrolysis waste heat to the urban central heating system (config by sector.biochar.heat_output).
The config.default.yaml contains the main parameters for calculating the CDR potential (application_per_sqkm, max_land_usage, number_years, under a top-level biochar: block), and sets sector.biochar.enable (and sector.biochar.heat_output) to false by default.

NOTE on Technology-data version

Data dependency note (technology-data)

add_biochar() requires a "biochar pyrolysis" cost entry with biomass-input, electricity-input, heat-output, capital_cost, and VOM parameters. Unlike enhanced rock weathering's data (already released in technology-data v0.15.0), biochar's cost data is not yet part of any released technology-data version, not v0.14.0 (currently pinned as "latest supported" in data/versions.csv), nor v0.15.0. It is currently only available in the PR PyPSA/technology-data#258.

Hence, running with sector.biochar.enable: true today will raise a KeyError.

Checklist

Required:

• [ OK] Changes are tested locally and behave as expected.
• [ OK] Code and workflow changes are documented.
• [ OK] A release note entry is added to doc/release_notes.md.
• [OK ] The description is human-written and any AI-generated content is marked.

If applicable:

• [OK ] Changes in configuration options are reflected in scripts/lib/validation.
• For new data sources or versions, these instructions have been followed.
• [ OK] New rules are documented in the appropriate doc/*.md files.

[BertoGBG]

@martavp @fneum