docs(friedland): add chapter 9 exhibit IV, V, VI and complete exhibits I-III

[priyam0k]

Summary of Changes

• added exhibit IV (us auto product mix study) to chapter 9 notebook using friedland_us_auto
• updated exhibit I, II and III tables with complete columns, plus sheets 3 and 4 for exhibit II
• updated assertions to check reported and paid ibnr across scenarios
• updated code to use implicit_axis=True for age extraction and clean .to_frame() series calls per feedback
• added exhibit V (sheets 1 and 2) for us pp auto using the gunnar benktander method
• added exhibit VI (sheets 1 and 2) for us auto using the gunnar benktander method
• add own distinct Column Notes block and total rows across all exhibits

Related GitHub Issue(s)

#588

Additional Context for Reviewers

keeping this PR in draft for now. will look into adding exhibit V and VI for chapter 9 covering unpaid claim estimates using the benktander method.

Checklist

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

---

Note

Low Risk
Documentation-only changes in a Friedland tutorial notebook; no library API or production reserving logic is modified.

Overview
Extends docs/friedland/chapter_9.ipynb so Friedland Chapter 9 now covers Exhibits I–VI: full BF projection and unpaid tables for industry auto, XYZ, and PP scenarios, plus Exhibit IV (U.S. auto product mix via friedland_us_auto) and Exhibits V & VI (Gunnar Benktander unpaid estimates for the PP and U.S. auto scenarios).

Exhibits I–III gain the missing table columns, additional sheets (e.g. Exhibit II sheets 3–4), per-exhibit Column Notes, add_total_row on displays, and reconciliation assert checks for reported/paid IBNR and related totals.

Notebook code is aligned with prior review feedback: accident-year age comes from latest_diagonal.to_frame(keepdims=True, implicit_axis=True, ...), and one-dimensional triangle extracts use consistent .to_frame(origin_as_datetime=False).squeeze() instead of ad hoc series handling.

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[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 14.9% of exported symbols fully typed (197 / 1323)

	Known	Ambiguous	Unknown	Total
Project (head)	197	111	1015	1323

Other symbols referenced but not exported by chainladder: 13

	Known	Ambiguous	Unknown	Total
Other (head)	3	1	9	13

Symbols without documentation:

• Functions without docstring: 316
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): no exported symbol type-completeness changes detected.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.47%. Comparing base (f7f08e8) to head (1c50ba7).
⚠️ Report is 28 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1197      +/-   ##
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+ Coverage   91.25%   91.47%   +0.22%     
==========================================
  Files          91       91              
  Lines        5395     5664     +269     
  Branches      688      766      +78     
==========================================
+ Hits         4923     5181     +258     
- Misses        338      343       +5     
- Partials      134      140       +6     

Flag	Coverage Δ
unittests	91.45% <ø> (+0.20%)	⬆️

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