chore: stabilize CI and clarify roadmap

[adaamko]

Summary

• make all 85 core unit tests network-free and run the complete suite in CI on the declared minimum Python version;
• replace Hub-dependent tokenizer tests with a tiny local tokenizer and update the stale taxonomy schema expectation;
• align contributor commands, Python requirements, release links, and current-version documentation;
• clarify LettuceDetect's span-level grounding-verification thesis and scope benchmark claims by model, split, and metric;
• distinguish mmBERT's 0.642 unified span-F1 from its 0.528 RAGTruth-slice span-F1;
• update the public roadmap for the renamed/split milestones, correctness issues, claimed work, and GitHub Discussions.

This PR does not change detector inference behavior. Chunk aggregation and the HTTP contract remain design/correctness work in #74 and #75.

Closes #73.

Test evidence

HF_HOME=<empty> HF_HUB_OFFLINE=1 TRANSFORMERS_OFFLINE=1 python -m pytest
85 passed

python -m ruff format --no-cache --check --diff lettucedetect/ lettucedetect_api/ tests/
57 files already formatted

python -m ruff check --no-cache lettucedetect/ lettucedetect_api/ tests/ --extend-exclude lettucedetect/integrations/
All checks passed

Contribution rights

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