v0.256.0: the box sends its own sentence in the household's language (R-558 Part B)

MinAgent: 0.131.0 (unchanged). Needs hub v0.118.0+, which shipped first and
tolerates a box that sends none of this - every box in the fleet is that box
until this release reaches it.

The hub writes a household's e-mails in their language now, but about a third
of those mails carry a sentence the BOX composed, naming a drive, an app or a
number. The hub cannot translate one. So the box sends it twice.

- message_customer on POST /api/v1/event, omitempty. A HUNGARIAN household
  sends nothing extra at all, so its payload stays byte-for-byte what every box
  sends today and the hub's fallback path keeps being the one production
  exercises rather than a branch nobody takes.
- 19 producers render both sentences from ONE bundle key. `message` stays
  Hungarian always: it is what the operator is mailed and what the hub logs.
- customer.language bootstraps a new box - stored choice, then config, then
  Hungarian. The config value is NEVER written into settings.json: that would
  record a choice the household never made.

The Hungarian did not move, measured twice: the wire golden from the slice-2
base commit, and the Go parity gate over all 19 new keys.

Three guards had to learn the change and one caught me: the test seam now
carries the new field; the R-329 severity register reported two dynamic sites
as no longer existing the moment they moved off PushEvent (the walk now checks
36 severity literals, up from 20); and TestConfigLanguageIsWiredInMain reads
main.go, because cmd/ is gitignored and ripgrep does not.

A mistake, named: the first pass dropped displayName from three producers,
which would have mailed customers "Alkalmazás telepítve: %!s(MISSING)". Caught
reading the diff; now pinned by a test that refuses %!/MISSING/%s/%d in either
language.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_0159rPz1ZhFKsS53msqPYxtS

controller/README.md

@@ -3614,7 +3614,7 @@ without reaching `Images()`.
 
 ---
 
-### 18. Dashboard language (i18n) (v0.247.0–v0.254.0)
+### 18. Dashboard language (i18n) (v0.247.0–v0.256.0)
 
 Design: `felhom.eu/documentation/architecture/10-localisation.md`. Inventory:
 `felhom.eu/documentation/audits/I18N-INVENTORY-2026-09-17.md`.
@@ -3820,6 +3820,36 @@ controller/
 
 ## Configuration
 
+
+#### 18.5 The e-mails the hub sends (v0.256.0, slice 3)
+
+**The hub writes a household's e-mails in the household's language** from hub v0.118.0. It learns
+which language from the box: every report carries `"language"` (since v0.247.0), and the hub prefers
+the last reported one over the operator's creation-time default. Nothing here needs a setting.
+
+**The box sends about a third of those sentences itself**, and the hub cannot translate one — it
+arrives as finished text naming a drive, an app or a number. So `POST /api/v1/event` carries
+**`message_customer`**: the same sentence in the household's language, beside the unchanged Hungarian
+`message`.
+
+- `message` is **always Hungarian**. It is what the operator is mailed, what the hub logs and what
+  `notification_log` records; a household switching language moves nothing an operator reads.
+- A **Hungarian household sends no second copy at all** (`omitempty`), so its payload is byte-for-byte
+  what every box has always sent.
+- 19 producers render both from ONE bundle key (`event.*`): health ×3, controller started/updated/
+  update-failed, storage disconnected/reconnected, backup target absent/restored, app deployed/
+  deploy-started/deploy-failed/removed, crossdrive ×2, disaster recovery ×2, db-dump completed.
+- An **untranslatable tail** (a docker error, a validator's sentence) is appended to both renderings.
+- **Not converted, deliberately:** the operator-tier types (the hub keeps them off the customer
+  channel) and the producers whose sentence is composed in another package and reaches the notifier
+  already finished. Those still send Hungarian only, so an English household can still see one
+  Hungarian line in some mails. Tracked as a row.
+
+**A new box starts in the language the operator picked.** The hub renders `customer.language` into
+`controller.yaml`; `GetLanguage` prefers the household's stored choice, then that, then Hungarian.
+The config value is **never written into `settings.json`** — persisting it would record a choice the
+household never made.
+
 ### Controller config (`controller.yaml`)
 
 Single YAML file per customer, infrastructure-only. Does **not** contain app-specific config.