Adopt pixi for environment management and modernize CI

[martin-springer]

Adds pixi for reproducible, lockfile-backed environment management and rewrites CI to use it. Also, adds Python 3.14 support, and introduces composable pip extras.

Changes

Environment management

• Replaced requirements.txt, requirements-min.txt, and docs/notebook_requirements.txt with pixi environments and a lockfile (pixi.lock).
• Simplified environment layout: core (bare), default (notebooks), dev (notebooks + test), dev-py310–dev-py314, and dev-min (minimum dependency versions).
• All shared environments pin Python 3.13 via a solve-group to avoid cross-contamination.

Pip extras

• Added composable optional-dependency groups: [notebooks], [test], [dev], [doc], and [all].

CI

• Rewrote pytest.yaml and nbval.yaml to use prefix-dev/setup-pixi.
• Added a validate-envs job that checks core, default, and dev environments resolve and import correctly.
• Removed the requirements.yaml workflow.
• Added Python 3.14 to the test matrix and package classifiers.

Other

• Bumped minimum arch version from 5.0 to 5.6.
• Updated README, Sphinx index, developer notes, and example notebook setup cells.
• Fixed a pre-existing broken link to the TrendAnalysis example in the Sphinx docs.

Checklist

• Code changes are covered by tests
• Code changes have been evaluated for compatibility/integration with TrendAnalysis
• New functions added to __init__.py
• API.rst is up to date, along with other sphinx docs pages
• Example notebooks are rerun and differences in results scrutinized
• Updated changelog

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.82%. Comparing base (cadebc6) to head (1798fb4).

Additional details and impacted files

@@                      Coverage Diff                       @@
##           migrate-NREL-to-NatLabRockies     #493   +/-   ##
==============================================================
  Coverage                          96.82%   96.82%           
==============================================================
  Files                                 12       12           
  Lines                               2332     2332           
==============================================================
  Hits                                2258     2258           
  Misses                                74       74           

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