Interface: PresenceHandle

PresenceIndex — the pure presence sensor (D18).

Registration observes MOUNTED, nothing more. This index is deliberately a plain refcounted data structure that knows nothing about sessions, trees, routers, or footprint — the meaning of presence (dormancy below the router, overlay masking, tab exclusivity, assumed-active defaults) lives one layer up in NavSession, which COMPOSES this with the authored tree.

Contract points that make React StrictMode/HMR safe by construction:

Handles are identities: open() returns a token, close(token) is idempotent per token. setup→cleanup→setup nets to one open handle.
Instance handles (repeats containers) are tracked separately and are EXCLUDED from the fingerprint — a scrolling virtualized list must never look like world motion (that scoping rule is enforced here, at the lowest layer that can).
Visibility is an EXPLICIT signal store (set by show()/setVisible()/the visible: mount option). No amount of mount-counting can see CSS; when no signal exists the layer above serves honesty markers instead of guessing.