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.

.gitignore

@@ -2,6 +2,7 @@
 # data once filled in — never commit it.
 /dist
 /engine/build
+/interpreter/build
 
 *.o
 core