Adopt idiomatic Go style without breaking the public API

[mjradwin]

Cosmetic and style cleanups flagged by staticcheck plus a couple of
real fixes uncovered along the way:

• Package and exported-symbol doc comments now start with the
  documented name (ST1000 / ST1020); convert /* */ doc comments on
  GetYahrzeit / GetBirthdayOrAnniversary / MonthFromName to // form so
  godoc renders them correctly.
• Drop the s "strings" import alias and use strings.ToLower
  directly.
• Guard the elapsedDays cache with a sync.RWMutex. The bare map was
  unsafe under concurrent access and would panic at runtime under -race
  for any caller that fanned date conversions across goroutines.
• Compile the Adar I/II disambiguation regexp once at package scope
  instead of on every MonthFromName call.
• Replace the float-based mod helper with integer Euclidean modulo
  inlined into Weekday(); same result, no floating-point round-trip.
• Drop unreachable else branches after return in MonthsInYear,
  DaysInMonth and elapsedDays0, and remove default: break / no-op
  break statements in MonthFromName's switches.
• Use fmt.Sprintf / fmt.Errorf for panic and error messages instead of
  string concatenation with strconv.Itoa.
• Rename the unexported hdJson struct to hdJSON (ST1003 initialism)
  and update the matching local variable in hdate_test.go.
• Remove the unreachable trailing "Nisan" entry from longMonthNames
  (indices 1..13 are the only ones the bounds check allows).
• Tighten heAdar / heMonthNames to fixed-size [2]string values.

Verified with gofmt, go vet, go test, go test -race, and
staticcheck -checks=all — all clean. go doc -all output is
byte-identical before and after, so the exported API is unchanged.