fix: align std.format edge cases with Python

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Motivation:
The official std.format(str, vals) documentation says string formatting follows the same rules as Python, and that the % operator is shorthand for std.format:
https://jsonnet.org/ref/stdlib.html#std-format

sjsonnet diverged from those Python-style rules in several formatting edge cases:

• floating-point formats normalized IEEE -0.0 to positive zero for %f, %e, %g, and %G
• dynamic * width/precision silently accepted fractional Jsonnet numbers through truncation
• negative dynamic width/precision did not consistently follow Python % behavior
• %s ignored precision, including dynamic precision from *
• %g exponent selection around powers of ten was sensitive to floating-point log10 roundoff

Modification:
This change keeps std.format anchored to the official Python-style contract instead of treating any single implementation as the specification.

• Preserve the IEEE sign bit of -0.0 for floating-point conversions while leaving integer conversions unchanged.
• Normalize dynamic * width and precision through a shared helper:
  • Jsonnet numbers used for * must be finite whole-number values.
  • Negative dynamic width becomes left adjustment with the absolute width.
  • Negative dynamic precision becomes zero; %g then maps precision zero to one as Python does.
  • Fractional dynamic width/precision now raises a user-facing format error instead of being rounded or truncated.
• Apply %s precision by Jsonnet codepoint before width padding.
• Stabilize %g exponent selection around powers of ten by compensating for small log10 floating-point error.
• Add regression tests for negative zero, %g boundary values, dynamic * width/precision, %s precision, and fractional * errors.

Result:
sjsonnet now matches Python % behavior for the covered std.format edge cases. The table below includes the old sjsonnet behavior from master before this PR.

Expression	Python %	sjsonnet before this PR	sjsonnet after this PR	C++ jsonnet / go-jsonnet observed	jrsonnet observed
"%+f" % (-0.0)	"-0.000000"	"+0.000000"	"-0.000000"	"+0.000000"	"+0.000000"
"%f" % (-0.0)	"-0.000000"	"0.000000"	"-0.000000"	"0.000000"	"0.000000"
"%g" % (-0.0)	"-0"	"0"	"-0"	"0"	"0e-00"
"%g" % 0.0001	"0.0001"	"0.0001"	"0.0001"	"0.0001"	"0.0"
"%*g" % [-4, 1.0]	"1 "	"1"	"1 "	"1"	"1"
"%.*g" % [-3, 1.23456]	"1"	Internal Error, caused by / by zero	"1"	"5001e+00"	panics / overflows in the observed CLI build
"%.*s" % [-3, "abcdef"]	""	"abcdef"	""	"abcdef"	"abcdef"
"%.1s" % "😀x"	"😀"	"😀x"	"😀"	"😀x"	"😀x"
"%*g" % [3.7, 1.0]	TypeError: * wants int	" 1"	format error	" 1"	" 1"
"%.*g" % [2.7, 1.23456]	TypeError: * wants int	"1.2"	format error	"1.11.8812766372727552"	"1.2"

The PR is now a single format-only commit and no longer includes unrelated manifestXmlJsonml changes.

References:

• Official Jsonnet std.format documentation: https://jsonnet.org/ref/stdlib.html#std-format
• Python printf-style string formatting: https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting