ADR-0019: pluralisation is a translator concern, not deferred work

Per follow-up review: §8.5's framing read as "we'll do this
properly later". Reword to make it explicit that real
plural-form rules per locale (Fluent / ICU) are NOT a goal of
this project. Translators handle pluralisation in their
wording (`(s)` shorthand or rephrased templates) — sufficient
for a teaching tool's output surface, and we're not planning
to revisit it.

Matching Out-of-Scope entry tightened the same way.

docs/adr/0019-friendly-error-layer-and-i18n.md

@@ -369,19 +369,28 @@ up the value in the supplied keyword arguments, and inserts it.
 Unknown placeholders → translation error (caught at test time
 when the catalog is exercised).
 
-#### 8.5 Pluralisation — `(s)` suffix for now
+#### 8.5 Pluralisation — translator's choice, not a system feature
 
 The existing codebase uses the
 `"{count} row(s) inserted"` style. The catalog continues this.
-Real plural-form rules per locale (Fluent / ICU MessageFormat
-have zero / one / few / many / two / other tagged forms,
-language-specific) are explicitly **deferred**. Adding them is
-a meaningful design lift, not a drop-in upgrade, and the H1
-scope here is already wide enough.
 
-A future "advanced i18n" ADR would address pluralisation,
-gendered forms, ordinals, and date/number formatting hooks.
-This ADR documents that the limitation is known and intentional.
+Pluralisation is **a translator concern, not a system
+concern**. Placeholders only carry values (§8.4); the wording
+around them is fully under the translator's control. Languages
+that have an equivalent of the `(s)` shorthand can use it;
+languages that don't can rephrase to avoid the question
+entirely (`"applied to {count} row(s)"` →
+`"applied to {count} rows in total"` works for every count).
+For a teaching tool with a narrow output surface this is
+sufficient.
+
+Real plural-form rules per locale (Fluent / ICU MessageFormat,
+with zero / one / few / many / two / other tagged forms) are
+**explicitly not a goal of this project**. They would let a
+template supply separate singular and plural strings indexed
+by the same count, which is technically nicer but not
+materially better for the kind of output this app produces.
+We are not going to add them.
 
 #### 8.6 Validation
 
@@ -518,7 +527,9 @@ These are deliberately deferred to keep H1 shippable:
    ADR-0002. Today's H1 does not handle this path.
 3. **Settings persistence for the `messages` command.** Lives in
    a future settings ADR.
-4. **Plural-form rules per locale.** §8.5.
+4. **Plural-form rules per locale.** §8.5 — explicitly not a
+   goal, not just deferred. Translations handle pluralisation
+   in their wording (`(s)` shorthand or rephrased).
 5. **Runtime locale selection.** §8.2.
 6. **Locale-aware value formatting.** §8.7 — explicitly rejected,
    not just deferred.