Various friedland fix

[henrydingliu]

Summary of Changes

• added some missing friedland data from chapter 11
• consolidated xyz_auto_bi data
• changing Chapter 6 tables to using the consolidated xyz_auto_bi dataset

Related GitHub Issue(s)

closes #926
closes #925
closes #924

Additional Context for Reviewers

• I passed tests locally for both code (uv run pytest) and documentation changes (uv run jb build docs --builder=custom --custom-builder=doctest)

---

Note

Low Risk
Changes are limited to bundled sample CSVs, _manifest.py metadata, and Friedland documentation doctests; no runtime estimation or core triangle logic is modified.

Overview
This PR enriches bundled Friedland sample data and simplifies how Chapter 6 loads triangles, without changing library APIs.

friedland_xyz_auto_bi is expanded in _manifest.py and its CSV so one sample carries paid/reported losses plus claim counts, case outstanding, and reported severities. The separate friedland_xyz_case and friedland_xyz_freq_sev entries and CSVs are removed; that data now lives only on the consolidated friedland_xyz_auto_bi sample.

friedland_auto_freq_sev gains a Paid Claims column (mostly populated on latest development rows). friedland_wc_self_insurer gains Payroll on calendar-year rows, supporting chapter 11-style examples.

docs/friedland/chapter_6.rst doctests no longer load friedland_xyz_freq_sev for count/severity tables; they slice the same tri from friedland_xyz_auto_bi for reported/closed counts and severity/case-outstanding calculations.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.28%. Comparing base (31b3432) to head (cc4756f).

Additional details and impacted files

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##             main     #927   +/-   ##
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  Coverage   88.28%   88.28%           
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  Files          88       88           
  Lines        5029     5029           
  Branches      642      642           
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  Hits         4440     4440           
  Misses        444      444           
  Partials      145      145           

Flag	Coverage Δ
unittests	88.28% <ø> (ø)

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