feat(plan): register the plan session-projection unit

plan-mode contributes the RFC's complete double-event example: command/run
records named plan set the wanted target (off -> false, else true),
plan/mode commits and clears it, and view derives {active, pending}.
Pending is thereby a pure replay quantity recovered from the log alone.
The plan key merges into SessionProjectionMap from src/types.ts, served
through the new ./types and ./client outlets (session-title template);
compositions without the registry are unaffected.

packages/plan/plan-mode/README.md

@@ -18,6 +18,10 @@ When `ctx.commands` is composed, the package registers `/plan [message]` and res
 
 The TUI consumes the plugin-owned `/plan` command; other front doors may drive the same service directly without defining a second mode vocabulary.
 
+## Session projection
+
+When the composition mounts `ctx.sessionProjections` ([`@deepseek-ai/dsh-session-projection`](../../session-projection/session-projection/README.md)), this package registers the `plan` projection unit under an injected child. The unit folds two event kinds: a `command/run` record named `plan` sets the wanted target (`off` → inactive, anything else → active), and `plan/mode` commits the logged state and clears it; every other event returns the same state reference. `view` derives `{ active, pending }`, where `pending` is true only while an outstanding selection differs from the logged state — a pure replay quantity, so host restarts, other tabs, and cold reads all recover it from the log alone (the `/plan` handler calls `set()` before any failing path, keeping the logged request and the run plane from forking). The key merges into `SessionProjectionMap` from `src/types.ts` (served to host consumers via `./types` and client aggregates via `./client`); the framework drives the unit and carriers serve the value on the history tail page and the `session/projection` push frame. Compositions without the registry are unaffected.
+
 ## Configuration
 
 ```yaml