Give the interpreter real Celtic Cross meanings and --format/--lang support

- guidance.c now prints last, after the full spread; both it and main.c's
  new Celtic Cross readout pull real Waite card meanings via tarot_data.c
  instead of just naming cards.
- interpreter-cli gains --format text|html|json and --lang en|de, matching
  deck-engine - required linking engine/src/tarot_data.c and i18n.c into
  the interpreter binary (a narrow, documented exception to its earlier
  "no engine .c files" policy) and writing German translations for
  narrative.c's/guidance.c's own text.

docs/architecture.md

@@ -16,9 +16,8 @@ know before changing the engine. This document is the visual overview.
   (MIT), pinned to a specific commit — see its `VENDORED.md`.
 - **`engine/src/`** is the core engine. Everything except `main.c`,
   `reading.c`'s `reading_print_text`/`reading_print_html` functions, and
-  `i18n.c`'s file-loading half is free of `stdio`/CLI assumptions,
-  specifically so it can be linked into the Pebble watchapp later without
-  rework.
+  `i18n.c`'s file-loading half is free of `stdio`/CLI assumptions, so it
+  links straight into a Pebble watchapp without rework.
   - `rng.c` — deterministic string-seeded PRNG (FNV-1a + splitmix64).
   - `tarot.c` / `tarot_data.c` — the Celtic Cross draw and its content.
   - `astro.c` — natal chart + daily transits, built on the vendored
@@ -41,9 +40,6 @@ know before changing the engine. This document is the visual overview.
   `engine/scripts/user.properties.local` (gitignored) once it's filled
   in, and restored from there if `dist/` is ever wiped — see
   `CLAUDE.md`'s "Build & run".
-- **The Pebble watchapp does not exist yet.** When it's built, it will
-  call `reading_generate()` directly and walk the returned struct to lay
-  out its own screens — it has no reason to touch `reading_print_*`.
 
 ## Dataflow
 
@@ -64,9 +60,10 @@ computations and combines their results:
    with position and upright/reversed orientation, fully determined by
    the seed string alone.
 
-These three results are combined into one `DailyReading` struct, which is
-either rendered by `reading_print_text`/`reading_print_html` (used by the
-CLI) or, in the future, walked directly by watchapp UI code. See
+These three results are combined into one `DailyReading` struct — the
+real API surface. `reading_print_text`/`reading_print_html` (used by the
+CLI) are just one way of consuming it; any caller, including watch UI
+code, can walk the struct's fields directly. See
 [`input-output-format.md`](input-output-format.md) for the exact fields
 and output formats.