Add economic cost overrides and dotted sweeps

[Str4vinci]

This makes BREOS's economic assumptions directly configurable and sweepable without requiring a throwaway cost preset for every price or CAPEX value. It delivers Phase 0 of the economic-scenario roadmap while keeping the existing flat-pricing model and preset results unchanged.

Summary

• add a validated [costs] override table with explicit precedence: user override → named preset → CostParams default
• use the existing cost-catalog vocabulary for electricity, export, standing, PV, battery, inverter, installation, land, maintenance, operation, and other costs
• centralize the public-key-to-CostParams mapping so App presets and lower-level config construction cannot drift
• support quoted and native TOML dotted sweep keys such as "costs.electricity_cost" and costs.storage_cost_per_kwh
• deep-copy each sweep configuration so nested overrides preserve sibling cost values and cannot mutate later runs
• reject unknown, empty, duplicate, and conflicting sweep keys with actionable available-key messages
• document the configuration and sweep workflow and update the shipped sweep example

Compatibility

Existing configurations need no changes. With no [costs] table, named presets and dataclass defaults resolve exactly as before; the residential Portugal preset is pinned character-for-character in a regression test. Sweep CSV columns retain the existing param_... convention, including dotted names such as param_costs.electricity_cost.

This is intentionally only the override seam. Escalator decomposition, scenario orchestration, switching values, currency, and time-of-use valuation remain in their roadmap phases.

Validation

• full non-slow suite: 812 passed, 7 skipped, 2 deselected
• focused App/config/CLI/economics suite: 188 passed
• Ruff lint and formatting checks passed
• strict offline Sphinx build with -W passed
• release artifact verification passed for the sdist, wheel, installed package data, and all 14 BLAST models