rt: keep 767 digits of a long number, so UCRT sees the sticky digit

scan_number kept 800 significant digits and stood for any nonzero digit
past them with one sticky 1, as the 801st. UCRT's strtod reads only 768
significant digits, so on Windows it never saw the sticky digit: a number
a hair above a halfway point between two DOUBLEs (2^53+1 followed by a
nonzero digit 1000 places on) rounded to even, down, and the rt test
scan_reads_a_number_of_any_length failed there.

767 digits are still exact -- no DOUBLE, nor any halfway point, has more
than 767 significant digits -- and the sticky digit is now the 768th.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

Author: jgarzik

rt/src/num.rs

@@ -135,11 +135,13 @@ const EXPONENT_MAX: i64 = 1 << 40;
 /// A number's significant digits, given to strtod as an integer and an
 /// exponent, so that a number of any length keeps its magnitude.
 ///
-/// 800 digits are enough for strtod to round exactly as it would with all of
+/// 767 digits are enough for strtod to round exactly as it would with all of
 /// them: no DOUBLE, nor any point halfway between two, has more than 767
-/// significant digits, and the digits past the 800th, when any is not 0, are
+/// significant digits, and the digits past the 767th, when any is not 0, are
 /// stood for by one sticky 1. So the value is what strtod gives the whole
-/// number.
+/// number. No more than that: UCRT's strtod reads only 768 significant
+/// digits, so a sticky digit any later is ignored there, and a number just
+/// past a halfway point rounded to even.
 struct Decimal {
     /// Sign, digits, sticky digit, 'e', exponent, NUL.
     buf: [u8; Decimal::CAP],
@@ -150,7 +152,7 @@ struct Decimal {
 }
 
 impl Decimal {
-    const DIGITS: usize = 800;
+    const DIGITS: usize = 767;
     const CAP: usize = Decimal::DIGITS + 32;
 
     const EMPTY: Decimal = Decimal {
@@ -192,7 +194,7 @@ impl Decimal {
         if self.len == 0 {
             self.put(&mut at, b'0'); // "-0" is -0, as strtod read it before
         } else {
-            // Past 100000 either way, 800 digits are infinity or 0 alike.
+            // Past 100000 either way, 767 digits are infinity or 0 alike.
             let exponent = exponent.clamp(-100_000, 100_000) - i64::from(self.sticky);
             if self.sticky {
                 self.put(&mut at, b'1');