Load example data from rheodata instead of bundling it

[marcocaggioni]

rheofit no longer bundles example data — it loads it from the rheodata package (pip install rheopy-rheodata), now live on PyPI at v0.1.0. One source of truth for data.

What changed

• Dependency: rheopy-rheodata>=0.1 added to pyproject.toml.
• Deleted bundled data: rheofit/data/*.json, rheofit/data/*.xlsx (4 files) and docs/walkthrough/*.json (3 files). All PNGs/SVGs in docs/walkthrough/ kept.
• Walkthrough docs rewritten to load via rheodata.to_rheofit(...):
  • walkthrough.md → caggioni_pg_carbopol_2pct / carbopol_2pct (CLI block replaced with the Python API; reproducibility now via seed=0)
  • walkthrough-carreau.md → caggioni_linear_polymer_flow / linear_polymer (flow-curve fit executable; amplitude/frequency sweeps from the same session remain in the comparison figures — those datasets are not in rheodata v0.1.0 yet)
  • walkthrough-carreau-carreau.md → caggioni_wlm_polymer_temp_series / T_18…
  • walkthrough-carbopol-glycerin.md → caggioni_carbopol_glycerin_temp / T_20,T_30,T_40 (also fixes the stale 0/1/2 step indices — sample IDs are explicit now)
• Demo reworked: demo_source() materializes the Carbopol demo dataset from rheodata as a TRIOS JSON in the temp dir — still no network needed (data ships in the rheodata wheel). CLI --demo and the skill demo flow keep working. DATA_DIR/DEMO_JSON_FILE removed; DEMO_SAMPLE_NAME is now "Carbopol 2% in PG (demo)".
• Skill (rheofit/skills/ and .github/skills/ copies): demo section rewritten around the rheodata Carbopol dataset (tc vs HB head-to-head); also drops the dead P&G SharePoint fallback URL.
• README: demo_source() description updated.

Verification

• Fresh pip install of the branch pulls rheopy-rheodata from PyPI; import rheofit works.
• Every walkthrough fit re-run end-to-end via rheodata — all documented numbers reproduce exactly: TC 6.018e-04 / HB 3.651e-03 (Carbopol PG), Carreau 3.847e-03 (linear polymer), TC 1.22e-04/2.05e-04/1.42e-04 at 40/30/20 °C (glycerin), carreau_carreau 8.278e-04 (WLM 18 °C).
• CLI python -m rheofit --demo (print_steps + tc fit) works on the materialized demo.
• sphinx-build -W -b html docs docs/_build/html: clean for all touched pages (only pre-existing myst.xref_missing warnings for the WASM explorer iframes, which are generated by export_interactive.py in CI).
• No test suite in CI (docs + app-deploy workflows only); no P&G references introduced.

Follow-up (not in this PR): add the linear-polymer amplitude/frequency sweeps to rheodata so the Carreau walkthrough is fully executable again.