v0.257.0: the app catalog can speak English (R-560, slice 5 Part A)

The READ PATH for a second language in `.felhom.yml`. An `i18n: {en: …}` sibling
block inside the same file; `Metadata.For(lang)` merges it FIELD BY FIELD over the
Hungarian, so a missing or blank English field shows the Hungarian one and a
half-translated app is a legal, shippable state.

`For("hu")` is the parsed struct with `I18n` cleared and nothing else — measured
against all 53 real catalog files, copied into `internal/stacks/testdata/catalog/`.
Lists replace whole; every other list is matched by its own key, never by position.
`For` never writes through the receiver: the metadata is the stack manager's, shared
by concurrent requests, and an in-place merge would leak one household's language
into another household's page.

Pages reach catalog copy only through `LocalizeStacks`/`LocalizeStackPtr`/`MetaFor`,
and `TestNoDirectMetaCopyReadOnPages` keeps a named, reasoned allow-list of every
direct `.Meta.<copy>` read in `internal/web` so the NEXT page to read one fails the
suite instead of quietly rendering Hungarian to an English household.

Eight red-proofs. Two of them convicted a hollow TEST rather than the code: a struct
copy shares its slices' backing arrays, so the obvious DeepEqual mutation check
passed a deliberately broken merge; and a one-entry fixture cannot tell key matching
from position matching. Both rewritten, both then seen to fail.

MinAgent: 0.131.0 (unchanged). Older controllers are unaffected — `LoadMetadata`
uses non-strict `yaml.Unmarshal`, so a pre-0.257.0 box drops the whole block.

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

controller/internal/stacks/testdata/catalog/audiobookshelf/.felhom.yml

@@ -0,0 +1,86 @@
+# =============================================================================
+# .felhom.yml - App metadata for felhom-controller
+# =============================================================================
+
+# --- Display info (shown on dashboard) ---
+display_name: "Audiobookshelf"
+description: "Hangoskönyv és podcast kezelő szerver"
+category: "media"
+subdomain: "audiobooks"
+slug: "audiobookshelf"
+# catalog_since: the date THIS repo last changed this app's pinned images. Any commit that
+# changes an image: line must set this to the same day (see CLAUDE.md).
+catalog_since: "2026-07-18"
+
+# --- Resource hints (displayed on deploy screen) ---
+resources:
+  mem_request: "100M"
+  mem_limit: "512M"
+  pi_compatible: true
+  needs_hdd: true
+
+# --- Backup classification (referential coupling; see felhom.eu documentation/audits/SPIKE-backup-classification-2026-07-14.md) ---
+backup:
+  userdata:
+    - path: media/audiobooks
+      class: optional          # PENDING-VETO: spike chose excluded; ruled optional for consistency
+                               # with komga/romm (curated, often personally ripped — precious).
+                               # If Viktor rules excluded, flip THIS LINE ONLY before committing.
+    - path: media/podcasts
+      class: excluded          # re-downloadable by definition
+
+# --- Deploy fields (first deployment only) ---
+deploy_fields:
+  - env_var: DOMAIN
+    label: "Domain"
+    type: domain
+    description: "A szerver domain neve"
+    locked_after_deploy: true
+
+  - env_var: SUBDOMAIN
+    label: "Aldomain"
+    type: subdomain
+    default: "audiobooks"
+    required: true
+    locked_after_deploy: true
+    description: "Az alkalmazás aldomainje"
+
+  - env_var: HDD_PATH
+    label: "Hangoskönyv tár útvonal"
+    type: path
+    required: true
+    placeholder: "/mnt/felhom-drives/hdd_1"
+    description: "A külső merevlemez elérési útja"
+    locked_after_deploy: true
+
+# --- App info (info page content) ---
+app_info:
+  tagline: "Hangoskönyv és podcast kezelő, lejátszó és szinkronizáló"
+  docs_url: "https://www.audiobookshelf.org/docs"
+
+  use_cases:
+    - 'Hangoskönyvek és podcastok rendszerezése és lejátszása'
+    - 'Automatikus metaadat letöltés - borítók, leírások, fejezetek'
+    - 'Folytatás ott, ahol abbahagytad (szinkron eszközök között)'
+    - 'Podcast feliratkozás és automatikus letöltés'
+    - 'Dedikált mobil alkalmazások (Android, iOS)'
+
+  first_steps:
+    - 'Nyisd meg az audiobooks.DOMAIN címet a böngészőben'
+    - 'Hozd létre az admin fiókot az első megnyitáskor'
+    - 'Add hozzá a könyvtárakat (/audiobooks és/vagy /podcasts)'
+    - 'Várd meg az automatikus szkennelést'
+    - 'Telepítsd az Audiobookshelf alkalmazást a telefonodra'
+
+  prerequisites:
+    - 'Külső HDD szükséges a hangoskönyvek tárolásához'
+    - 'Hangoskönyvek mappákba rendezve (pl. Szerző/Könyv/)'
+
+# --- Controller-side health probe ---
+healthcheck:
+  checks:
+    - type: api
+      port: 80
+      path: "/healthcheck"
+      expect:
+        status: 200