docs: document v44.3.0 implementation status and dense-storage limitation

- Add "Ce qui a été implémenté — v44.3.0" section: Variable.as_of
  attribute, Holder changes, mitigated risks, 14 tests, known gaps
- Add "Limite de l'implémentation actuelle : stockage dense" section:
  explains why reference sharing is ineffective for scattered individual
  changes (e.g. civil servants changing grade), quantifies the memory
  gap vs patch-based storage, and sketches the patch storage model
  that v_next should implement

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: benjello