Add an ultimate label alias, global and per-Triangle

[Abhayindia]

Adds ULT_LABEL as a global option plus a per-Triangle ult_label that overrides it, as discussed on the issue.

The alias is applied at display time only - _repr_format, which __repr__, _repr_html_ and to_frame all go through - so ddims and .development keep the ULT_VAL/9999 sentinel and nothing downstream changes. That matters for things like utility_functions.py's run-off filter, which string-compares development labels against a valuation date.

>>> ult = cl.Chainladder().fit(cl.load_sample('raa')).ultimate_
>>> list(ult.to_frame(origin_as_datetime=False).columns)
['2261']
>>> cl.options.set_option('ULT_LABEL', 'Ultimate')
>>> list(ult.to_frame(origin_as_datetime=False).columns)
['Ultimate']
>>> override = ult.copy(); override.ult_label = 'ULT'
>>> list(override.to_frame(origin_as_datetime=False).columns)
['ULT']
>>> list(ult.to_frame(origin_as_datetime=False).columns)   # global untouched
['Ultimate']
>>> ult.development.tolist()                               # sentinel intact
['2261']

It covers the 9999 development-lag case too, e.g. full_expectation_ ends [..., 132, 'Ultimate'].

I left the default as None so nothing changes unless you opt in - happy to default it to "Ultimate" if that's the [BRK] you had in mind, it's a one-line change.

pytest chainladder gives 1142 passed. The 2 remaining failures are test_heatmap_render, which fails the same way on a clean checkout here (no IPython in my venv).

Closes #1345.

[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 (209 / 1382)

	Known	Ambiguous	Unknown	Total
Project (head)	209	111	1062	1382

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: 327
• Functions without default param: 0
• Classes without docstring: 10

Patch (exported symbols added or changed by this PR): 50.0% fully typed (1 / 2)

	Known	Ambiguous	Unknown	Total
Patch	1	0	1	2

Patch symbol details

Symbol	Status	Change
chainladder.core.triangle.Triangle.ult_label	❌ unknown	new
chainladder.utils.tests.test_utilities.test_ult_label	✅ known	new

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 91.78%. Comparing base (6b51f64) to head (1b64076).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1351      +/-   ##
==========================================
+ Coverage   91.76%   91.78%   +0.02%     
==========================================
  Files          96       96              
  Lines        5475     5490      +15     
  Branches      706      708       +2     
==========================================
+ Hits         5024     5039      +15     
  Misses        327      327              
  Partials      124      124              

Flag	Coverage Δ
unittests	91.78% <100.00%> (+0.02%)	⬆️

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[Abhayindia]

saw @henrydingliu's note on #1345 - i built this to the spec genedan gave there, so no attachment to it either way. happy to close if the team lands on not wanting it.