fix(parallelogram): drop the daily lookback window by date, not row count

[SAY-5]

Summary of Changes

With approximation_grain="D", parallelogram_olf removed the lookback window with iloc[366 * lookback_years + leap_day:]. When the lookback year itself is a leap year (a triangle starting 2017 with a 2016 lookback), 366 rows lands on 2016-12-31 rather than 2017-01-01, so the non-leap frame keeps a one-day 2016 origin the leap frame does not have, the positional join misaligns the two, and fit() fails with ValueError: operands could not be broadcast together with shapes (1,1,3,1) (1,1,4,1). The lookback is now dropped by date (index >= first_date), which is independent of leap days and leaves the same origins in both frames. The existing daily-grain expectations are unchanged.

Related GitHub Issue(s)

Fixes #1219

Additional Context for Reviewers

Added test_daily_grain_after_leap_year with the repro from the issue.

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)

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.1% of exported symbols fully typed (202 / 1337)

	Known	Ambiguous	Unknown	Total
Project (head)	202	111	1024	1337

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: 324
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 0.0% fully typed (0 / 1)

	Known	Ambiguous	Unknown	Total
Patch	0	0	1	1

Patch symbol details

Symbol	Status	Change
chainladder.adjustments.tests.test_parallelogram.test_daily_grain_after_leap_year	❌ unknown	new

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.27%. Comparing base (606be5c) to head (a11ab4a).
⚠️ Report is 20 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1221   +/-   ##
=======================================
  Coverage   91.27%   91.27%           
=======================================
  Files          91       91           
  Lines        5411     5412    +1     
  Branches      692      692           
=======================================
+ Hits         4939     4940    +1     
  Misses        338      338           
  Partials      134      134           

Flag	Coverage Δ
unittests	91.27% <100.00%> (+<0.01%)	⬆️

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[priyam0k]

@SAY-5 Welcome to the repo and thanks for putting this PR together so quickly.

[priyam0k]

The solution is correct. Let maintainers have their final review.