std.format %e/%g do not follow documented Python rounding semantics

[He-Pin]

std.format is documented as following Python formatting rules:
https://jsonnet.org/ref/stdlib.html#std-format

In jsonnet v0.22.0, %e/%g do not renormalize the mantissa when rounding carries from 9... to 10.... %g precision 0 also appears to be passed through as a negative fractional precision, while Python treats precision 0 for %g as precision 1.

Repro table:

Expression	Python 3	C++ jsonnet v0.22.0	go-jsonnet v0.22.0	jrsonnet v0.5.0-pre99
"%.0g" % 1.0	"1"	"5e+00"	"5e+00"	"0e+00"
"%+.0g" % 1.0	"+1"	"+5e+00"	"+5e+00"	"+0e+00"
"%.0g" % 0.1	"0.1"	"5"	"5"	"0"
"%.1g" % 9.9	"1e+01"	"10"	"10"	"10"
"%.2g" % 99.9	"1e+02"	"100"	"100"	"100"
"%.0e" % 9.5e10	"1e+11"	"10e+10"	"10e+10"	"10e+10"
"%.1e" % 9.99	"1.0e+01"	"10.0e+00"	"10.0e+00"	"10.0e+00"

Commands used:

python3 - <<PY
for expr in [1.0, 0.1, 9.9, 99.9, 9.5e10]:
    pass
print("%.0g" % 1.0)
print("%+.0g" % 1.0)
print("%.0g" % 0.1)
print("%.1g" % 9.9)
print("%.2g" % 99.9)
print("%.0e" % 9.5e10)
print("%.1e" % 9.99)
PY

jsonnet -e '["%.0g" % 1.0, "%+.0g" % 1.0, "%.0g" % 0.1, "%.1g" % 9.9, "%.2g" % 99.9, "%.0e" % 9.5e10, "%.1e" % 9.99]'

Likely cause:

In stdlib/std.jsonnet, %g calls render_float_dec/sci with fpprec - 1, so %.0g passes -1 precision into the renderer. Separately, the renderer rounds the mantissa but does not normalize 10 * 10^precision back to 1 * 10^precision with exponent + 1.

This is related in spirit to #879, but that issue was about fixed-point carry for %f-style formatting. This issue covers %e/%g scientific renormalization and %g precision-zero behavior.

[He-Pin]

I re-ran these against Python directly to avoid relying only on the docs wording.

Environment:

Python 3.14.5

Python old-style % formatting results:

"%.0g" % 1.0   => "1"
"%.0g" % 0.0   => "0"
"%.0g" % 0.1   => "0.1"
"%#.0g" % 1.0  => "1."
"%.1g" % 9.9   => "1e+01"
"%.2g" % 99.9  => "1e+02"
"%.0e" % 9.5e10 => "1e+11"
"%.1e" % 9.99 => "1.0e+01"

Current Jsonnet v0.22.0 C++ and go-jsonnet both produce the non-Python results for these cases, e.g. "%.0g" % 1.0 gives "5e+00", "%.1g" % 9.9 gives "10", and "%.0e" % 9.5e10 gives "10e+10".