Allow Triangle.drop() to drop origin levels

[priyam0k]

Summary of Changes

Implements origin-level dropping in Triangle.drop(), so origin periods can be removed via origin= or axis=2 / axis='origin', building on the axis routing added in #1131.

• tri.drop(origin='1990') and tri.drop(labels='1990', axis=2) are equivalent
• dropping reduces the origin axis through the existing odims slicing path, so odims and values stay in sync
• only the first or last origin period(s) may be dropped, so the triangle stays contiguous; dropping an interior period raises ValueError
• unknown origin labels raise KeyError
• added tests for dropping the first/last period, the axis equivalents, the interior guard, and the missing-label case
• added a doctest example to the drop() docstring

Related GitHub Issue(s)

closes #1055, part of #1052

Additional Context for Reviewers

• the origin / axis=2 path introduced in Add index, columns, origin, development alternatives to Triangle.drop() #1131 previously raised NotImplementedError; this PR implements it. The corresponding test was updated to only assert NotImplementedError for the still-unimplemented index and development axes

• the "first or last only" rule is enforced by checking that the kept origin positions remain contiguous, which rejects interior drops that would leave a gap

• development-axis dropping (Allow Triangle.drop() to drop development periods #1057) is intended as a follow-up using the same approach

• I passed tests locally for code (pytest)

---

Note

Low Risk
Localized API extension in Triangle.drop() with explicit validation and thorough tests; no auth, persistence, or broad behavioral changes outside origin slicing.

Overview
Triangle.drop() now supports removing origin periods via origin= or axis=2 / axis='origin', alongside the existing column axis. Dropping uses boolean origin slicing so odims and values stay aligned.

Only first or last origin label(s) can be removed; interior drops that would leave a gap raise ValueError. Missing labels raise KeyError. Label handling is broadened with pd.api.types.is_list_like (including native pd.Period labels).

Docs add an origin-drop doctest; tests cover first/last drop, axis equivalents, interior guard, and missing labels. Index and development axes still raise NotImplementedError.

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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 (195 / 1307)

	Known	Ambiguous	Unknown	Total
Project (head)	195	110	1002	1307

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: 314
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 14.3% fully typed (1 / 7)

	Known	Ambiguous	Unknown	Total
Patch	1	0	6	7

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.origin_tri	✅ known	new
chainladder.core.tests.test_triangle.test_drop_origin_axis_equivalents	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_origin_first	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_origin_interior_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_origin_last	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_origin_missing_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_drop_origin_period_label	❌ unknown	new

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.97%. Comparing base (a9f45e1) to head (82eb1a9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1163      +/-   ##
==========================================
+ Coverage   90.94%   90.97%   +0.02%     
==========================================
  Files          91       91              
  Lines        5325     5339      +14     
  Branches      675      678       +3     
==========================================
+ Hits         4843     4857      +14     
  Misses        344      344              
  Partials      138      138              

Flag	Coverage Δ
unittests	90.97% <100.00%> (+0.02%)	⬆️

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