Replace Rational() method calls with rational literals for performance

Replace Rational(1, 2) with 0.5r and Rational(0) with 0r across
core.rb, datetime.rb, and strftime.rb. Rational literals are resolved
at compile time, avoiding method dispatch overhead on every call.

Note: Rational(4800001, 2) in amjd is intentionally kept as-is because
4800001/2r is parsed as Integer#/(Rational) division at runtime, which
is 2x slower than the direct Rational() constructor.

Benchmark (Ruby 4.0.1 +YJIT, benchmark-ips):

  Date.new!:
    Before (Rational(1,2)):  2.812M i/s (355.58 ns/i)
    After  (0.5r):           3.596M i/s (278.11 ns/i)
    Improvement:             +27.9%

  Date#day_fraction:
    Before (Rational(0)):   11.230M i/s  (89.05 ns/i)
    After  (0r):            29.947M i/s  (33.39 ns/i)
    Improvement:            +166.7%

  Micro-benchmark (literal vs method call):
    0r   vs Rational(0):    34.5M vs 11.3M i/s (+3.05x)
    0.5r vs Rational(1,2):  34.2M vs  8.8M i/s (+3.90x)

Author: jinroq

lib/date/core.rb

@@ -328,7 +328,7 @@ def _load(s)
     def new!(ajd = 0, of = 0, sg = DEFAULT_SG)
       # ajd is Astronomical Julian Day (may be Rational)
       # Convert to integer JD and day fraction (same as C's old_to_new)
-      raw_jd = ajd + Rational(1, 2)
+      raw_jd = ajd + 0.5r
       jd = raw_jd.floor
       df = raw_jd - jd
       obj = allocate
@@ -875,7 +875,7 @@ def cwyear
   #   DateTime.new(2001,2,3,12).day_fraction # => (1/2)
   #
   def day_fraction
-    @df || Rational(0)
+    @df || 0r
   end
 
   # call-seq:
@@ -1585,7 +1585,7 @@ def marshal_load(array)
     when 3
       # Format 1.8: [ajd, of, sg]
       ajd, _of, sg = array
-      raw_jd = ajd + Rational(1, 2)
+      raw_jd = ajd + 0.5r
       jd = raw_jd.floor
       df = raw_jd - jd
       init_from_jd(jd, sg, df == 0 ? nil : df)

lib/date/datetime.rb

@@ -140,7 +140,7 @@ def ajd
     time_r = Rational(@hour * 3600 + @min * 60 + @sec_i, 86400) +
              Rational(@sec_frac.numerator, @sec_frac.denominator * 86400)
     of_r = Rational(@of, 86400)
-    jd_r + time_r - of_r - Rational(1, 2)
+    jd_r + time_r - of_r - 0.5r
   end
 
   # ---------------------------------------------------------------------------
@@ -440,12 +440,12 @@ def marshal_load(array)
     when 2
       jd_like, sg_or_bool = array
       sg = sg_or_bool == true ? ITALY : (sg_or_bool == false ? JULIAN : sg_or_bool.to_f)
-      _init_datetime(jd_like.to_i, 0, 0, 0, Rational(0), 0, sg)
+      _init_datetime(jd_like.to_i, 0, 0, 0, 0r, 0, sg)
     when 3
       ajd, of_r, sg = array
       of_sec = (of_r * 86400).to_i
       # Reconstruct local JD and time from AJD
-      local_r = ajd + Rational(1, 2) + of_r
+      local_r = ajd + 0.5r + of_r
       jd      = local_r.floor
       rem_r   = (local_r - jd) * 86400
       h       = rem_r.to_i / 3600
@@ -857,7 +857,7 @@ def _dt_new_by_frags(hash, sg)
         warn("invalid offset is ignored: #{of}", uplevel: 0)
         of = 0
       end
-      sf = hash[:sec_fraction] || Rational(0)
+      sf = hash[:sec_fraction] || 0r
       _new_dt_from_jd_time(jd, h, m, s, sf, of, sg)
     end
 
@@ -880,7 +880,7 @@ def _split_sec(second)
         s_i = s_r.floor
         [s_i, s_r - s_i]
       else
-        [Integer(second), Rational(0)]
+        [Integer(second), 0r]
       end
     end
 
@@ -932,7 +932,7 @@ def _split_second(second)
       s_i = s_r.floor
       [s_i, s_r - s_i]
     else
-      [Integer(second), Rational(0)]
+      [Integer(second), 0r]
     end
   end
 

lib/date/strftime.rb

@@ -523,7 +523,7 @@ def internal_sec
   end
 
   def sec_frac
-    Rational(0)
+    0r
   end
 
   def of_seconds