ludic.i18n: a language's lookups kept once read; switching back restores them

i18n_use re-read and re-parsed the .po on every change of language (ev_SettingsChanged, SetLang)
and dropped the old tables for good: the play of 22 minutes left lines_of 650 KB, fs_read_text
409 KB, PoEntry 113 KB and unquote 273 KB unreachable, and three of its @alloc_ok sites were
declared but unbounded. Each language's tables are now filed by index once built (tables.ludic,
I18nTables) and put back by reference; a new probe (i18n_init) or a different file for a code
(lang_add) reads it again.

Test: a language switched back to is its kept tables, not its file read again; a new probe reads
it (9 tests pass).

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/ludic.i18n/ports.ludic

@@ -44,6 +44,7 @@ export state I18nState {
   plural: Dict = null                 # a plural msgid (the singular) -> index into forms / 3
   forms: []string = null              # three forms per entry, "" where the file has fewer
   plural_rule: int = 0                # PL_*
+  tabs: []I18nTables = null           # by language: its lookups once read, kept for the next switch
 }
 
 # where the shipped languages are (a directory with languages.txt and <code>.po), and the atlas a