broker: add stopped-preserving upgrade without actor execution

PROTOCOL.md

@@ -125,7 +125,7 @@ the request: the request's `type` in past tense (`spawned`, `upgraded`,
 | `type` | Fields | Effect |
 | --- | --- | --- |
 | `spawn` | `actor`, `wasm`, `source`?, `built_by`? | test, then start a new actor |
-| `upgrade` | `actor`, `wasm`, `source`?, `built_by`? | test, then swap the version |
+| `upgrade` | `actor`, `wasm`, `source`?, `built_by`?, `preserve_stopped`? (boolean, default `false`) | test, then swap the version; with `preserve_stopped: true`, a stopped actor is compiled and installed without execution and stays stopped |
 | `rollback` | `actor`, `version` | go back to an older version |
 | `stop`, `start` | `actor` | stop or start an actor |
 | `remove` | `actor` | stop the actor (normal shutdown), then delete its whole folder `/actor/<id>/` (all versions, `data/`, `public/`, logs); answer `{"type": "removed", "actor"}`. Messages to it are then undeliverable, and the ID can be spawned again. An unknown actor or a native one (`broker`, `console`, `cargo`, `fs`) is an `error` |
@@ -182,10 +182,11 @@ change of `path` or a path under it (`/actor/a` covers `/actor/a/x`, not
   restarts and upgrades. `unwatch` with the same path ends it.
 
 **Versions.** A new version (from `spawn`, `upgrade`, or a changed
-`actor.wasm` on disk) is tested first, while the old version keeps running.
+`actor.wasm` on disk) is tested first, while the old version keeps running
+(except the stopped-preserving upgrade described below).
 Only when its tests pass does the broker copy it to `versions/<n>/`
 (`actor.wasm`, `source/`, `meta.json`) and switch to it. So `versions/` holds
-exactly the versions that were installed, and instances always run from
+exactly the versions that were installed (including stopped installations), and instances always run from
 there. If the tests fail, nothing changes, and the sender gets an `error`
 with the reason. All versions share the actor's `data/` and `public/`: a
 new version must read the data of the old one.
@@ -214,6 +215,21 @@ instance. A version without test files passes.
 top-level `actor.wasm` and `source/` a copy of the new version, and starts
 it. A rollback does the same with the older version.
 
+**Stopped-preserving upgrade.** Send `upgrade` with `"preserve_stopped": true`
+to install a stopped actor's new program without starting it, running `main`
+(even as a candidate), or reading or sending any of its mail. The host compiles
+the WASM and copies the new version, source and metadata as usual, increments
+the version, and keeps `actor.json`'s `running: false`. Its mailbox, `data/`
+and `public/` stay untouched. Because candidate tests execute `main`, they are
+not run in this case: the `upgraded` reply contains
+`"tests": {"skipped": "preserve_stopped"}`. Test the build separately before
+installing it this way. An explicit `start` later runs the installed version
+and delivers the queued mail. No startup/crash probation runs for this stopped
+installation. Running actors still use the normal tested swap, even with this
+option. Omitting the option (or `false`) retains the existing behavior,
+including starting a stopped actor. Explicit stop/start requests serialize
+with upgrades of the same actor.
+
 **Crashes.** The broker restarts a crashed actor after 0.5 s, then doubles
 the delay up to 30 s. A new version that crashes 3 times within 30 seconds
 after an upgrade goes back to the previous version. The actor that asked for