One unit definition already serves every session — its cells are keyed by
`Session` — but registrants became per-session when agent presets started
mounting tool packages per agent. N sessions on one preset register the
same key N times.
The first registration won and owned the only disposer, so ending one
session stripped `goal`, `todos`, `plan`, `tokenUsage` and
`contextPressure` from every other live session's snapshot. Measured
against the shipped `standard` preset: two concurrent sessions each had
eight projection keys, and disposing the first left the second with three
— the ones host rows register.
Count the registrants instead and remove the key when the last one goes.
A differing `stateVersion` still refuses to share: it is the one
incompatibility a runtime comparison can name, since everything else
about a definition is functions.
Yichen Jiang
fedb8a2702