Add JSON output format and a separate interpreter binary

deck-engine gains --format json (language-independent, slug-based)
so a new dist/interpreter-cli can score how significant a day's
transits are without linking the engine itself — it depends only on
the JSON shape, via its own minimal parser. Also documents the full
reading pipeline end to end (CLAUDE.md, README, docs/) and adds
docs/reading.md, a non-technical explanation of what a daily reading
contains and means.

CLAUDE.md

@@ -33,7 +33,7 @@ Two ways to run a reading:
 dist/deck-engine --seed "2026-07-03" \
   --birth-date 1990-05-14 --birth-time 14:32 --birth-utc-offset 2 \
   --birth-lat 52.5200 --birth-lon 13.4050 \
-  [--date YYYY-MM-DDTHH:MM] [--format text|html] [--lang en|de] [--i18n-dir <path>]
+  [--date YYYY-MM-DDTHH:MM] [--format text|html|json] [--lang en|de] [--i18n-dir <path>]
 
 # 2. dist/run.sh [text|html] [lang] — wraps the binary for daily use: seed
 #    and --date come from the OS clock (today's date / current UTC time),
@@ -76,6 +76,7 @@ deck_in_a_dash/
     tests/smoke_test.c
     scripts/run.sh, scripts/user.properties.template
     Makefile
+  interpreter/                  Separate module + binary; see "Interpretation" below.
   dist/                         Build output (gitignored-style; see Build & run).
 ```
 
@@ -181,6 +182,73 @@ into the Pebble watchapp unchanged later. Only `reading_print_text`/
 `_html` (text formatting), `main.c` (CLI arg parsing), and `i18n_load`
 (reads a file from disk) are desktop-only and won't port as-is.
 
+## Interpretation (`interpreter/`)
+
+A second, separate top-level module and **its own binary**
+(`dist/interpreter-cli`, sibling to `dist/deck-engine`) that scores *how
+significant a day's transits are* — the first piece of a larger "turn the
+raw reading into an actual narrative" effort described in project chat
+history, not yet producing any narrative text. `deck-engine` itself is
+deliberately unchanged by this beyond gaining `--format json`; the two
+binaries compose over that JSON, never by linking together:
+
+```bash
+cd engine      && make          # -> dist/deck-engine (unchanged; --format json is new)
+cd interpreter && make          # -> dist/interpreter-cli
+cd interpreter && make test     # builds and runs both interpreter/tests/*_test.c
+
+dist/deck-engine ... --format json | dist/interpreter-cli
+# or: dist/deck-engine ... --format json > reading.json && dist/interpreter-cli reading.json
+```
+
+- `interpret_daily_reading(reading, &out)` (`significance.h/.c`) scores
+  every aspect in `reading->transits.aspects[]`, keeps the **top 5** by
+  score in `DailyInterpretation.top_items[]` (descending, evicting the
+  rest), and classifies the day into a `DaySignificance` enum (`DAY_QUIET`
+  ... `DAY_MAJOR`) from the single highest-scoring item.
+  `interpretation_deserves_framing()` is true only at `DAY_MAJOR` — the
+  intended hook for giving the Celtic Cross reading a "this matters"
+  intro sentence naming `top_items[0]` on days that earn it; that
+  rendering doesn't exist yet, `main.c`'s report is plain text only.
+- Score = `transiting-planet weight (slow/outer planets score higher) ×
+  orb tightness (1.0 = exact, ~0 = at the edge) × natal-luminary bonus
+  (transits to natal Sun/Moon score higher than to other planets)`. The
+  weights and the `DAY_QUIET`/`DAY_NOTABLE`/`DAY_SIGNIFICANT`/`DAY_MAJOR`
+  thresholds in `significance.c` are hand-tuned, not derived from
+  anything physical — expect to retune them once real readings are
+  compared against how "major" a day actually feels.
+- **`reading_print_json`** (`engine/src/reading.c`) is the wire format:
+  the full `DailyReading` (natal, transits, spread), using the
+  language-independent `astro_*_slug()`/`tarot_*_slug()` accessors
+  (`astro.h`/`tarot.h`) rather than `astro_*_name()`/`tarot_*_name()` —
+  the JSON must stay identical regardless of `--lang`, since it's a
+  machine interchange format, not display text.
+- **`json.c`/`json.h`** is a small hand-rolled recursive-descent JSON
+  parser (object/array/string/number/bool/null) - not a general-purpose
+  validating library, just enough to parse `deck-engine`'s own output.
+  Unlike the core engine, it uses `malloc`; `interpreter-cli` was never
+  meant to run on the watch itself.
+- **`reading_io.c`** walks the parsed JSON down to `"transits"."aspects"`
+  and fills a `DailyReading` with just that (everything else is left
+  zeroed - `interpret_daily_reading` doesn't read `natal`/`spread`
+  either). An aspect naming a planet/aspect-type slug it doesn't
+  recognize, or a document missing `"transits"."aspects"` entirely (as
+  opposed to a present-but-empty array, which is a valid "no aspects
+  today"), makes the whole load fail rather than silently dropping data.
+- **Deliberately header-only dependency on the engine, throughout**:
+  `significance.h`/`reading_io.h` `#include engine/src/reading.h` for the
+  struct/enum *definitions*, but `interpreter/Makefile` never compiles or
+  links any engine `.c` file (or the vendored Astronomy Engine) — every
+  test in `interpreter/tests/` runs against hand-built JSON text or
+  hand-built `DailyReading` values, with no real ephemeris/tarot-draw
+  call involved anywhere. Consequently `reading_io.c` (planet/aspect
+  slugs) and `main.c` (planet/aspect display names) each duplicate a
+  small local table rather than linking `astro.c` to reuse its own -
+  keep those in sync if the engine's slugs/names ever change.
+- Only `Aspect`s compete for the top 5 so far (`SignificantItemKind` is
+  currently just `ITEM_ASPECT`); moon phase and house ingresses are
+  candidate future item kinds but aren't scored yet.
+
 ## Licensing
 
 - Engine code (`engine/src/`, `engine/scripts/`, `engine/Makefile`):