Add persistent index support to CTable

 - New CTableIndex handle with col_name, kind, name, stale properties
 - create_index(), drop_index(), rebuild_index(), compact_index() methods
 - index() lookup and indexes property on CTable
 - _CTableIndexProxy duck-type shim routes sidecar files to
   <table.b2d>/_indexes/<col_name>/ for persistent tables
 - Index catalog stored in /_meta vlmeta; survives table close/reopen
 - where() automatically uses a fresh index; falls back to scan when stale
 - Epoch tracking: mutations (append, extend, setitem, assign, sort_by,
   compact) mark all indexes stale; delete() bumps visibility_epoch only
 - Views raise ValueError for all index management methods
 - Add _indexes to reserved column names in schema_compiler
 - 32 new tests in tests/ctable/test_ctable_indexing.py
 - New example examples/ctable/indexing.py
 - New tutorial doc/getting_started/tutorials/15.indexing-ctables.ipynb

 Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: FrancescAlted