Add cross-provider run comparisons

[AbdelStark]

Problem

Preserved eval and benchmark runs could be compared only with a shallow row model. That made cross-provider deltas ambiguous and did not separate real performance differences from capability, fixture, budget, or suite mismatches.

Solution

• Add a shared comparison context for preserved run reports covering providers, capabilities, operations, fixture digests, budget refs, suite versions, missing evidence, skip reasons, event counts, and claim boundaries.
• Refuse incompatible comparisons with explicit mismatch details while allowing compatible cross-provider rows.
• Extend JSON, Markdown, and CSV comparison exports with the new context and budget status fields.
• Add direct report-model tests plus public docs, changelog, and roadmap updates.

Closes #150

Validation

• uv lock --check
• uv run ruff check src tests examples scripts
• uv run ruff format --check src tests examples scripts
• uv run python scripts/generate_provider_docs.py --check
• uv run pytest tests/test_harness_report_compare.py tests/test_benchmark.py tests/test_evaluation_suites.py
• uv run pytest tests/test_harness_report_compare.py tests/test_harness_workspace.py tests/test_docs_site.py -q
• uv run mkdocs build --strict
• uv run pytest
• uv run --extra harness pytest --cov=src/worldforge --cov-report=term-missing --cov-fail-under=90
• bash scripts/test_package.sh
• uv build --out-dir dist --clear --no-build-logs
• git diff --check