docs: fix DevelopmentConstant patterns dict in friedland chapter_8

[SaguaroDev]

What

The final code cell of docs/friedland/chapter_8.ipynb errors during the docs build, failing the Doctest CI job:

cl.DevelopmentConstant(patterns=patterns, style='cdf').fit(auto_bi["Paid Claims"]).ldf_

KeyError: 12
  File chainladder/development/constant.py:78, in DevelopmentConstant.fit
    ldf = xp.array([float(self.patterns[item]) for item in obj.ddims])

Why

patterns is defined as a nested dict:

patterns = {
    'reported': {12: 4.0, 24: 2.9, ...},
    'paid':     {12: 90.0, 24: 15.0, ...},
}

But the non-callable patterns path in DevelopmentConstant.fit (and the docstring: "A dictionary key:value representation of age(in months):value") expects a flat {age: value} dict. fit indexes self.patterns[item] for each item in obj.ddims ([12, 24, ...]), so with the nested dict it looks up patterns[12] and raises KeyError.

Fix

Pass the 'paid' sub-dict in the fit cell, since it fits auto_bi["Paid Claims"]. The nested patterns definition is left intact for the preceding cell (which shows the full reported+paid pattern set). One-line change to the notebook source.

Verification

Executed the notebook end-to-end locally (jupyter nbconvert --execute, 120s timeout) — it now completes with no CellExecutionError, and the fit cell produces a clean execute_result with the expected LDFs (12-24: 6.0, 24-36: 3.0, ... matching the Friedland exhibit).

The bug is present on both main and experimental; targeting main.

---

Note

Low Risk
Documentation-only notebook one-liner; no library or production code paths change.

Overview
Fixes a docs notebook cell that broke Doctest / nbconvert execution: the Friedland chapter 8 example passed the full nested patterns dict into DevelopmentConstant.fit on Paid Claims, but the non-callable patterns path indexes {age_in_months: value} directly (self.patterns[item] for each obj.ddims entry), which raised KeyError: 12.

The change passes patterns['paid'] so the fit uses the paid CDF pattern that matches auto_bi["Paid Claims"], while the earlier cell can still display the nested reported+paid pattern set.

^{Reviewed by Cursor Bugbot for commit de9b0dc. Bugbot is set up for automated code reviews on this repo. Configure here.}

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.90%. Comparing base (8a9a46e) to head (de9b0dc).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #897   +/-   ##
=======================================
  Coverage   86.90%   86.90%           
=======================================
  Files          87       87           
  Lines        4932     4932           
  Branches      624      624           
=======================================
  Hits         4286     4286           
  Misses        456      456           
  Partials      190      190           

Flag	Coverage Δ
unittests	86.90% <ø> (ø)

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[henrydingliu]

kenneth is actively working on this. this may conflict with his complete version. mind keeping this as draft for now?