feat: grid carbon-intensity keys in the site config (hpc-campaign p4) #208

Overview

PyAutoNerves half of hpc-campaign phase 4 (PyAutoFit#1746). Adds grid carbon-intensity values to the site.yaml schema so autofit.campaign.carbon reads site constants from config, never code.

Plan

• site_config.py: an optional carbon_intensity table, keyed by basis (regional, national, desnz), each a low/central/high SiteValue in gCO2e/kWh with unverified support; ci_basis names the central one
• site/ral.yaml: South England regional (central 188, p5 81, p95 308 g/kWh), national 2025 (129; 51–231), DESNZ 2025 (177), all unverified: true, with sources in comments (report §1.4)
• Tests: schema validation, defaults when absent
• Tier: judge — merge mode: human /prm

Heart RED at the door (2026-10-30); Human override for Heart RED (development only) authorized live by the human for hpc-campaign phases 3 and 4. Merge still needs green CI and /prm; no release.

Suggested branch: feature/hpc-campaign-p4-carbon-report