feat(nav): each module declares its URL keys; a page switch clears them and a page opens without another's

The shell cleared a hand-kept key list on a page switch (it missed the Files and feed keys) and cleared
nothing when a page opened from a link carrying another page's keys. ModuleDef.urlKeys now names every
key a page's view writes, the shell derives both from the registry, and moduleUrl.contract.test holds the
declarations true: one owner per key, no shell key, exactly what each codec writes, and no undeclared
key literal in the app.

frontend/src/features/navigation/moduleRegistry.ts

@@ -48,6 +48,17 @@ export const REGISTERED_PAGES: readonly Page[] = ALL.map((m) => m.id);
 
 const BY_ID = Object.fromEntries(ALL.map((m) => [m.id, m])) as Record<Page, ModuleDef>;
 
+// Every module's address-bar keys, and those of every module but `page`: what a page switch clears and
+// what a page opens without (NavigationProvider). Derived from the definitions, so a new module's keys
+// are cleared the day it declares them.
+const ALL_URL_KEYS: readonly string[] = ALL.flatMap((m) => m.urlKeys);
+export function moduleUrlKeys(): readonly string[] {
+  return ALL_URL_KEYS;
+}
+export function foreignUrlKeys(page: Page): readonly string[] {
+  return ALL.filter((m) => m.id !== page).flatMap((m) => m.urlKeys);
+}
+
 // Look up a module's definition by page id (every Page is covered, incl. settings).
 export function moduleDef(id: Page): ModuleDef {
   return BY_ID[id];