fix: correct %g and %e exponent for powers of 10

[He-Pin]

Motivation

%g % 1000000 produces "1000000" instead of "1e+06", and %.2g % 1000 produces "10e+02" instead of "1e+03". C printf and Python both produce the correct output.

Root cause: std.log(x)/std.log(10) in the standard library suffers from floating-point imprecision for powers of 10:

• log(1000000)/log(10) = 5.9999999999999991 (should be 6)
• log(1000)/log(10) = 2.9999999999999996 (should be 3)

std.floor then rounds these down incorrectly.

Modification

Added 1e-12 epsilon before std.floor in the exponent calculation in two places in cpp-jsonnet/stdlib/std.jsonnet:

1. render_float_sci (used by %e/%E)
2. %g/%G format handler

Regenerated astgen/stdast.go from the updated standard library.

The epsilon is ~1000x the floating-point error (~1e-15) but far too small to affect non-power-of-10 values.

Result

Expression	Before	After	C/Python
"%g" % 1000000	"1000000"	"1e+06"	"1e+06"
"%.2g" % 1000	"10e+02"	"1e+03"	"1e+03"
"%e" % 1000000	"1.000000e+05" (wrong)	"1.000000e+06"	"1.000000e+06"
"%G" % 1000000	"10E+05"	"1E+06"	"1E+06"

All other %g/%e cases produce identical output before and after this fix.

Test plan

• go test ./... — all tests pass
• Verified against C printf and Python %g for: 0, ±1000000, 1e10, 1e-10, 123456, 999999, 3.14, various precision specifiers
• No regression in existing format tests

Related

• Upstream PR: fix: correct %g and %e exponent for powers of 10 jsonnet#1324 (same fix in cpp-jsonnet/stdlib/std.jsonnet)
• Once fix: correct %g and %e exponent for powers of 10 jsonnet#1324 is merged, the cpp-jsonnet submodule reference should be updated