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/metadata.go

@@ -66,6 +66,12 @@ type Metadata struct {
 	// with the Fork-3 asymmetry — a malformed PATH rejects the whole block, an unknown ROLE drops
 	// just that entry (see ValidateDataPaths).
 	DataPaths []DataPath `yaml:"data_paths,omitempty" json:"data_paths,omitempty"`
+	// I18n is the catalog's copy in every language OTHER than the Hungarian above (R-560). Keyed by
+	// language code; only "en" exists today. It is read through `For(lang)` and NEVER rendered
+	// directly — see metadata_i18n.go for the whole contract. `json:"-"`: this struct is serialised
+	// into the API answers and the page data, and the untranslated half of a bundle has no reader
+	// there; the merged view is what travels.
+	I18n map[string]MetadataOverlay `yaml:"i18n,omitempty" json:"-"`
 }
 
 // SMTPMapping renames the generic relay settings (host / port / security / from / from-name)