incomplete cholesky handling, preconditioner guidance, error handling

[vt2211]

Changes

• make ichol the public compiled incomplete-Cholesky name
• keep ic only as a compatibility alias
• keep ichol0 separate
• keep the old Python IC(0) path clearly separated as native_ic / native_ichol
• improve missing-dependency handling for ilupp
• improve sparse preconditioner failure diagnostics and practitioner guidance
• update tests to cover the alias normalization and fallback behavior

[codecov]

Codecov Report

❌ Patch coverage is 69.60784% with 31 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.54%. Comparing base (2a7b99b) to head (6979988).

Files with missing lines	Patch %	Lines
fvgp/gp_lin_alg.py	59.09%	27 Missing ⚠️
fvgp/gp_kv.py	50.00%	2 Missing ⚠️
tests/test_fvgp.py	93.75%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #33      +/-   ##
==========================================
- Coverage   81.88%   81.54%   -0.35%     
==========================================
  Files          18       18              
  Lines        5482     5558      +76     
==========================================
+ Hits         4489     4532      +43     
- Misses        993     1026      +33     

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.