fix: Align stdlib 1:1 edge semantics

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Motivation

This follows a 1:1 stdlib audit against local google/jsonnet stdlib/std.jsonnet and Jsonnet 0.22.0. Several native stdlib implementations in sjsonnet diverged on edge semantics while still needing to stay efficient on hot paths.

Modification

• Add official-style hidden comparison helpers and shared comparison logic for Jsonnet stdlib semantics.
• Align minArray/maxArray default sentinels, explicit keyF=false / onEmpty=false, and non-comparable value errors.
• Align array/string helpers including mapWithIndex, flattenArrays, flattenDeepArray, repeat, removeAt, isEmpty, string escaping helpers, and resolvePath.
• Preserve explicit keyF=false behavior in set helpers without treating it as the default sentinel.
• Add focused direction tests covering the audited compatibility cases and update the existing isEmpty golden.

Result

• git diff --check
• ./mill --no-server 'sjsonnet.jvm[3.3.7].checkFormat'
• ./mill --no-server 'sjsonnet.jvm[3.3.7].test' (141/141 passed)

Follow-up Issues

Not mixed into this PR because they are higher-risk or need compatibility policy confirmation:

• std.makeArray fractional size differs from Jsonnet 0.22.0 #796 std.makeArray fractional sizes.
• std.slice fractional semantics differ from Jsonnet 0.22.0 #797 std.slice fractional index/step behavior.
• std.setUnion string inputs differ from Jsonnet 0.22.0 #798 std.setUnion string inputs.
• manifestIni and manifestToml differ from Jsonnet 0.22.0 std.jsonnet #799 manifestIni / manifestToml strictness and formatting.

Base64 non-ASCII and substr fractional semantics remain tracked separately in #793 and #795.

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Additional audit update after adding source-built jrsonnet to the comparison matrix.

Environment:

• official C++ Jsonnet: Jsonnet commandline interpreter v0.22.0
• go-jsonnet: Jsonnet commandline interpreter (Go implementation) v0.22.0
• jrsonnet: source-built jrsonnet 0.5.0-pre98, commit 80cd36abd868507312e2cc2c78cb0f55a684c620
• sjsonnet: this PR's worktree assembly

Summary:

• The cases fixed in this PR still look correct against official Jsonnet: removeAt fractional/negative indexes, minArray/maxArray non-comparable values, isEmpty over object/function, non-string escape helpers, keyF=false empty/singleton behavior, flatten*, repeat, mapWithIndex on strings, and filterMap laziness.
• go-jsonnet and jrsonnet are useful cross-checks, but both diverge from official on several stdlib edge cases. We should not use either as the sole compatibility oracle.
• Merge/stacking matters: this PR's worktree does not include all parallel stdlib fixes, so a final integration branch should include fix: Ignore hidden fields in std.mapWithKey #792 and fix: Align stdlib edge cases with Jsonnet #794 or rebase after them.

Notable remaining/parallel items:

item	official behavior	go-jsonnet	jrsonnet	sjsonnet PR #800	coverage
std.mapWithKey hidden fields	ignores hidden fields	matches official	matches official	still maps hidden fields in this worktree	#792
std.substr("hello", 1.2, 2)	"el"	error	error	error	#795
std.makeArray(2.5, function(i) i)	[0, 1]	error	error	error	#796
std.slice([1,2,3], 1.2, 3, 1)	error	[2,3]	error	[2,3]	#797
std.setUnion("ab", "bc")	"[\"a\", \"b\"]c"	matches official	error	["a","b","c"]	#798
std.parseInt("+1")	error	1	error	1	covered by #794 parse rewrite
std.manifestToml({a:{b:"c"}})	"\n\n[a]\n b = \"c\""	matches official	"[a]\n b = \"c\""	"[a]\nb = \"c\""	#799
std.base64("é")	"6Q=="	"w6k="	"w6k="	"w6k="	#793 policy/tracking

This PR can remain focused, but before final merge we should avoid a partial stdlib state where #800 lands without the earlier hidden-field/parse fixes or without clear follow-up issues for the deliberate policy differences.

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Follow-up from the expanded stdlib audit pass (100 additional function-family cases across official C++ Jsonnet 0.22.0, go-jsonnet 0.22.0, source-built jrsonnet 0.5.0-pre98, and this PR's sjsonnet assembly):

New compatibility issues opened from that pass:

• std.clamp rejects comparable non-number values accepted by Jsonnet 0.22.0 #802 std.clamp rejects comparable non-number values accepted by official Jsonnet (std.clamp("b","a","c"), std.clamp([2],[1],[3])).
• std.splitLimit negative maxsplits differs from Jsonnet 0.22.0 #803 std.splitLimit / std.splitLimitR reject negative integer maxsplits values other than -1, while official C++ Jsonnet treats them as unlimited.
• std.reverse string inputs differ from Jsonnet 0.22.0 #804 std.reverse rejects string input, while official Jsonnet returns an array of reversed characters.

Already-covered differences observed in the same pass:

• std.asciiUpper / std.asciiLower, std.equalsIgnoreCase, and string std.setMember are covered by fix: Align stdlib edge cases with Jsonnet #794.
• std.mapWithKey hidden-field behavior is covered by fix: Ignore hidden fields in std.mapWithKey #792.

The broader pass did not change the assessment of this PR's core fixes: its included cases still match official behavior. The main integration concern remains merge/stack ordering so that #792/#794 and the follow-up issues are not lost from the overall stdlib alignment effort.