Add Triangle.ffill() to forward-fill along the origin/development axes

[Abhayindia]

Closes #1030.

Adds Triangle.ffill(axis), mirroring pandas' ffill but along the origin or development axis, and never writing a value into a cell that hasn't been valued yet.

On the semantics question I asked in #1030 and never got an answer to: genedan's own worked example in the issue answers it, once you check it against the triangle's actual nan_triangle. In his axis=3 sketch, 1987 at ages 36/48 stays missing even though 1300 precedes it - because those cells are beyond the triangle's evaluation date, not because they're a "later" NaN. His axis=4 example (there's no axis 4, so I read it as origin) is consistent with the same rule the whole way through: 1988 at ages 24-48 stays missing for the same reason. There's exactly one cell in the whole issue that breaks the pattern - 1986 at age 48 in the axis=3 example, which the sketch fills to 1100 even though that cell is also beyond the evaluation date. Given eleven other cells across both examples agree and only this one doesn't, I'm reading it as a typo in the hand-written sketch rather than the intended behavior, and implemented the self-consistent rule: fill forward within the observed triangle, leave everything outside it untouched.

Docstring example reuses that same triangle from the issue (with the corrected 1986/48 cell) for both axes. Tests cover the development and origin directions, that it returns a new object rather than mutating in place, and the axis validation.

$ pytest chainladder/core/tests/test_triangle.py -q
353 passed, 1 skipped in 813s

[github-actions]

Pyright Type Completeness

View the full pyright --verifytypes output for this commit

Project (full chainladder package, at this PR's head): 15.6% of exported symbols fully typed (216 / 1387)

	Known	Ambiguous	Unknown	Total
Project (head)	216	111	1060	1387

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: 325
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 60.0% fully typed (3 / 5)

	Known	Ambiguous	Unknown	Total
Patch	3	0	2	5

Patch symbol details

Symbol	Status	Change
chainladder.core.tests.test_triangle.test_ffill_development_axis	✅ known	new
chainladder.core.tests.test_triangle.test_ffill_does_not_mutate_original	✅ known	new
chainladder.core.tests.test_triangle.test_ffill_invalid_axis_raises	❌ unknown	new
chainladder.core.tests.test_triangle.test_ffill_origin_axis	✅ known	new
chainladder.core.triangle.Triangle.ffill	❌ unknown	new

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.70%. Comparing base (5df817f) to head (fdf0a10).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1302      +/-   ##
==========================================
+ Coverage   91.67%   91.70%   +0.03%     
==========================================
  Files          93       93              
  Lines        5441     5462      +21     
  Branches      702      704       +2     
==========================================
+ Hits         4988     5009      +21     
  Misses        328      328              
  Partials      125      125              

Flag	Coverage Δ
unittests	91.70% <100.00%> (+0.03%)	⬆️

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