Allow development to accept an age in months relative to origin

[Abhayindia]

Closes #1094 (the enhancement route agreed with @henrydingliu in that thread).

_to_datetime tries %Y%m, %Y, %Y-%m-%d and then falls through to a bare pd.to_datetime, which for a numeric column silently reads the values as nanoseconds since epoch. That collapsed age-lag columns like [12, 24, 36] to 1970-01-01, and the old guard just checked whether the result landed in 1970 - so a triangle whose development really is the year 1970 got the same error.

This replaces that with a check at the point of parsing: if none of the explicit formats matched and the column is numeric, it's an age in months. _init_development then resolves each value to the valuation date that many months after the start of that row's origin period. Ages are measured from the origin period start (using the constructor's own origin_grain, so mid-period origin dates and fiscal-year anchors line up), not the literal recorded origin date. Bare years still match %Y and are untouched. Non-numeric unparseable input still raises, with the same message as before. Semiannual grains only support the calendar Jan/Jul anchor; a non-calendar one raises a clear error rather than guessing.

Verified:

$ python -m pytest chainladder/core/tests/test_triangle.py -q
312 passed, 1 skipped in 100.59s

Full suite was 1091 passed / 8 skipped when I posted the branch on the issue.

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.0% of exported symbols fully typed (204 / 1357)

	Known	Ambiguous	Unknown	Total
Project (head)	204	111	1042	1357

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: 323
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 100.0% fully typed (3 / 3); 1 no longer exported

	Known	Ambiguous	Unknown	Total
Patch	3	0	0	3

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_set_development_age_in_months	✅ known	new
chainladder.core.tests.test_triangle.test_set_development_age_respects_mid_period_origin	✅ known	new
chainladder.core.tests.test_triangle.test_set_development_bare_years_unaffected_by_age_support	✅ known	new
chainladder.core.tests.test_triangle.test_set_development_age_instead_of_date_raises	—	no longer exported (was ✅ known)

[henrydingliu]

just unblocked tests. expecting ruff and codecov fails.

[codecov]

Codecov Report

❌ Patch coverage is 64.70588% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 91.23%. Comparing base (d182ae4) to head (87a5a8f).
⚠️ Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
chainladder/core/base.py	62.50%	4 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1229      +/-   ##
==========================================
- Coverage   91.31%   91.23%   -0.09%     
==========================================
  Files          91       92       +1     
  Lines        5434     5451      +17     
  Branches      700      704       +4     
==========================================
+ Hits         4962     4973      +11     
- Misses        336      340       +4     
- Partials      136      138       +2     

Flag	Coverage Δ
unittests	91.23% <64.70%> (-0.09%)	⬇️

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