fix: a time label's mantissa cannot reach its own radix

`format_time()` picked its sub-second rung from the raw value, so three
significant figures of a value just under a rung's top rounded to 1000 and
`%.3g` wrote that as an exponent: a one-second tick computed as 0.9999999 s
read `1e+03ms`. The rung is now chosen by the rounded mantissa, which is the
same 999.5 check `format_si()` already made for the SI prefixes, and a value
that rounds to a whole second leaves the sub-second branch entirely.

src/plot/ticks.cpp

@@ -98,7 +98,10 @@ void format_time(f64 seconds, char* buf, usize n) {
   const char* const sign = seconds < 0.0 ? "-" : "";
   const f64 t = std::fabs(seconds);
 
-  if (t < 1.0) {
+  // `0.9995` and not `1.0`: three significant figures of 0.9999 s are `1e+03ms`, so a value
+  // that *rounds* to a second belongs to the whole-second branch below, which writes it `1s`.
+  // The same spill one rung down is handled inside the ladder.
+  if (t < 0.9995) {
     // Sub-second: the prefix that keeps the mantissa in [1, 1000) -- the same rule the SI
     // formatter uses on a y axis, and the reading a zoomed-in window needs. A fixed
     // millisecond unit printed `0.005ms` where `5us` is what the axis is showing, and
@@ -119,8 +122,12 @@ void format_time(f64 seconds, char* buf, usize n) {
         {1.0e-9, "ns"},
         {1.0e-12, "ps"},
     };
+    // The rung is picked by the *rounded* mantissa, which is what `format_si`'s own 999.5
+    // check buys there: 0.9999 ms is `999.9µs` before rounding and `1e+03µs` after, so it is
+    // the millisecond rung's value and reads `1ms`. Testing the raw `t < u.scale` let the
+    // mantissa reach 1000 at every rung boundary.
     for (const auto& u : kUnits) {
-      if (t < u.scale) continue;
+      if (t < u.scale * 0.9995) continue;
       std::snprintf(buf, n, "%s%.3g%s", sign, t / u.scale, u.unit);
       return;
     }