Disposal rate method

[henrydingliu]

Summary of Changes

Implement the disposal rate method

Related GitHub Issue(s)

closes #931

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

Medium Risk
New reserving logic touches IBNR allocation and LDF derivation with required ultimate alignment; scope is additive with tests but actuarial correctness is business-critical.

Overview
Adds DisposalRate, a Friedland-style adjustment that forecasts future emergence as a share of an external ultimate passed via sample_weight (e.g. incurred ultimate to drive a paid payment pattern).

fit builds observed disposal rates from cumulative X / ultimate, applies TriangleWeight drop rules (with documented order that differs from Development for rank vs range drops), and fits a cumulative disposal_ / incremental incr_disposal_ pattern through DevelopmentBase. transform / fit_transform reallocate IBNR using that pattern and refresh ldf_ / full_triangle_; sample_weight is required on fit and transform.

DisposalRate is exported from chainladder.adjustments and the top-level package; public API tests include it. MethodBase.validate_weight is now a @staticmethod with type hints so DisposalRate can validate ultimates without a full method fit.

New tests cover Friedland sample fidelity, missing-weight errors, parity with volume-weighted Chainladder, and sparse-backend output.

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[codecov]

Codecov Report

❌ Patch coverage is 97.05882% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.31%. Comparing base (468f11a) to head (7459e8e).

Files with missing lines	Patch %	Lines
chainladder/adjustments/disposal.py	96.66%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1022      +/-   ##
==========================================
- Coverage   89.38%   89.31%   -0.07%     
==========================================
  Files          91       91              
  Lines        5266     5232      -34     
  Branches      678      666      -12     
==========================================
- Hits         4707     4673      -34     
  Misses        391      391              
  Partials      168      168              

Flag	Coverage Δ
unittests	89.31% <97.05%> (-0.07%)	⬇️

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Cursor Bugbot has reviewed your changes and found 1 potential issue.

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