Problem

When a CNavGroup that contains nested sub-groups is collapsed and then re-opened, all of its sub-groups appear closed — their previous open state is lost. This diverges from the vanilla @coreui/coreui navigation (the source of truth) and from @coreui/vue, where the nested sub-groups stay open on reopen.

Root cause

CSidebarNav tracked a single global openChain (the path of ids from the root to the deepest open group) and each CNavGroup derived its visibility from a prefix match against it. Collapsing a parent truncated openChain to the parent's chain, discarding all descendant ids; reopening set it back to just the parent's own chain — so the descendants were forgotten.

A single open path simply cannot represent "parent collapsed, but its children remembered as open".

Fix

Replace the single-path model with a per-level accordion, mirroring vanilla (groupsAutoCollapse) and the Vue implementation:

• CNavGroupContext now carries { activeId, setActiveId, openBranch } for the current level instead of an ancestor-id chain.
• CSidebarNav provides the root-level accordion state.
• Each CNavGroup keeps its own activeChildId for its children. That state lives on the always-mounted component, so collapsing an ancestor never discards it — reopening restores whichever child was open.
• Opening a group still collapses its siblings (accordion), and controlled mode (visible + onVisibleChange) is unchanged.
• CNavLink opens its whole ancestor branch via openBranch() when it becomes active (replaces the old setOpenChain(parentChain)).
• Removed the now-unused internal CSidebarNavContext (not part of the public API).

Tests

• New: reopening a parent restores the open state of its nested subgroups.
• New: opening an uncontrolled group collapses its sibling.
• All existing CNavGroup tests (controlled, locked, active-link-opens-parent, nested default-open) pass unchanged; snapshots unchanged (markup is identical).

No prop changes, so the API tables are unaffected.