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.

README.md

@@ -44,6 +44,10 @@ engine/
   scripts/                       run.sh and the user.properties template.
   Makefile
 dist/                            Build output (gitignored) — see below.
+interpreter/                     Separate module + binary (dist/interpreter-cli):
+                                  scores how significant a day's transits are,
+                                  reading deck-engine's --format json output
+                                  (see CLAUDE.md).
 docs/                            Architecture/dataflow diagrams, input/output format reference.
 ```
 
@@ -84,7 +88,16 @@ dist/run.sh html de       # override the language for this run
 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]
+  [--date YYYY-MM-DDTHH:MM] [--format text|html|json] [--lang en|de]
+```
+
+**Interpreter** (`interpreter/`, built separately with `cd interpreter && make`):
+scores how significant a day's transits are, reading `deck-engine`'s
+`--format json` output — the two binaries compose over that JSON, they're
+never linked together:
+
+```bash
+dist/deck-engine ... --format json | dist/interpreter-cli
 ```
 
 ## License