Add narrative text and a day-significance rank to the interpreter

Each of the day's top significant transits now gets a short sentence
describing what it classically means and which area of life its house
governs, condensed from Sepharial's Transits and Planetary Periods
(1920, public domain) plus a small table of traditional house
significations. The day-level line also shows its rank out of the 4
defined levels, e.g. "Notable (2/4)". Docs (CLAUDE.md, README,
docs/input-output-format.md, docs/reading.md) updated to match,
including a full worked example in docs/reading.md.

interpreter/src/significance.h

@@ -34,7 +34,10 @@ typedef enum {
   DAY_QUIET = 0,
   DAY_NOTABLE,
   DAY_SIGNIFICANT,
-  DAY_MAJOR
+  DAY_MAJOR,
+  DAY_SIGNIFICANCE_COUNT /* not a real level - the number of levels above,
+                          * for callers that want to show day_level as a
+                          * "rank out of N" (e.g. main.c's report). */
 } DaySignificance;
 
 typedef struct {