Retire the two modules independently; calm the README down

The native-support check looked only for native B2 restic support and, on
finding it, set the kill switch and disabled the whole patch. That was fine when
providers were the only thing here. With nested-dataset snapshots in the patch it
is wrong: TrueNAS is likely to ship one capability long before the other, and a
single all-or-nothing kill switch would silently take a still-needed module down
with the superseded one.

apply.sh now treats the patch as two independent modules:

  providers  b2.py + restic.py + the UI credential dropdown
             native when B2RcloneRemote carries a real get_restic_config()

  nested     plugins/cloud/{snapshot,crud}.py + cloud_backup/sync.py (opt-in)
             native when the "no further nesting" validation is gone from
             plugins/cloud/crud.py

Each is detected and skipped on its own. The kill switch fires only once BOTH are
done (native, or nested was never enabled). The UI patch belongs to providers and
is skipped with it. The deferred middlewared restart now fires when ANY
still-needed module landed -- keying it off providers alone would have left a
freshly-patched nested module on disk and never loaded on a native-B2 box.
hook_status.json reports each module with an active flag and a reason.

README: dropped the warning boxes and the disclaimer's fear-bulleting in favour
of plain statements, documented the two-module design and the per-module
auto-disable, and added a Development section disclosing AI assistance. The one
caveat kept, as a plain sentence rather than a banner: the mount --bind staging
step has not yet been exercised by a live backup run.

67 tests, ruff and shellcheck clean.

CHANGELOG.md

@@ -91,6 +91,28 @@
 
 ### Changed
 
+- **The patch is now two independent modules, and each retires on its own.**
+  Previously the native-support check looked only for native B2 restic support
+  and, on finding it, set the kill switch and disabled *everything*. With a
+  second capability in the patch that would silently take a still-needed module
+  down with the superseded one — TrueNAS is likely to ship one of these long
+  before the other.
+
+  `apply.sh` now detects each separately (`providers`: does `B2RcloneRemote`
+  carry a real `get_restic_config()`; `nested`: is the *"no further nesting"*
+  validation still in `plugins/cloud/crud.py`), skips just the superseded one,
+  and only sets the kill switch once **both** are done. The UI patch belongs to
+  `providers` and is skipped with it. The deferred middlewared restart now fires
+  when *any* still-needed module landed — keying it off `providers` alone would
+  have left a freshly-patched `nested` module on disk and never loaded on a
+  native-B2 box. `hook_status.json` reports each module with an `active` flag and
+  a reason.
+
+- README rewritten to be less alarmist: dropped the warning boxes and the
+  disclaimer's fear-bulleting in favour of plain statements, and documented the
+  two-module design. The one caveat kept as a plain sentence: the `mount --bind`
+  staging step has not yet been exercised by a live backup run.
+
 - Version strings in `install.sh`, `uninstall.sh`, and `recover.sh` were stale
   at `0.0.4`; all scripts now report the same version.
 - `patch_ui.py`: replaced a `try`/`except`/`pass` with `contextlib.suppress`