fix: Align stdlib edge cases with Jsonnet

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Motivation:
Several stdlib functions diverged from official Jsonnet on lazy callback evaluation, ASCII-only string helpers, string set membership, numeric parsing beyond Long range, and signed zero handling.

Modification:

• Preserve std.filterMap callback laziness and avoid evaluating elements that the callbacks do not force.
• Reject non-string results from std.flatMap on strings, including null.
• Make std.asciiUpper, std.asciiLower, and std.equalsIgnoreCase ASCII-only instead of JVM Unicode case folding.
• Allow std.setMember to operate on string sets with set validation enabled.
• Parse std.parseInt, std.parseOctal, and std.parseHex into Jsonnet number semantics instead of Long-bounded integers.
• Normalize std.sign(-0) to numeric zero.
• Add focused utest coverage plus new direction tests under new_test_suite.

Result:
sjsonnet now matches official Jsonnet v0.22.0 for these stdlib edge cases while preserving performance-oriented implementations for hot paths.

Validation:

• jsonnet --version -> Jsonnet commandline interpreter v0.22.0
• Official jsonnet on new success direction tests returned true: filter_map_laziness, ascii_case_non_ascii, equals_ignore_case_ascii, set_member_string, parse_large_numbers, sign_negative_zero.
• Official jsonnet on error.std.flatmap_string_null.jsonnet errors with std.flatMap on strings, provided function must return a string, got null.
• ./mill --no-server sjsonnet.jvm[3.3.7].test -> SUCCESS
• ./mill --no-server sjsonnet.jvm[3.3.7].checkFormat -> SUCCESS
• git diff --check and git diff --cached --check -> clean

Notes:

• Base64 non-ASCII string semantics are intentionally not changed in this PR and are tracked separately in Track std.base64 non-ASCII string semantics vs official Jsonnet #793.
• The parse tests assert numeric equality semantics like official Jsonnet. This PR does not change sjsonnet large-number manifestation formatting.

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Additional audit note from the four-implementation matrix:

expression	official Jsonnet 0.22.0	go-jsonnet 0.22.0	source-built jrsonnet 0.5.0-pre98 (80cd36a)	sjsonnet PR #800 worktree
std.parseInt("+1")	error	1	error	1
std.parseHex("+10")	error	error	error	16

The PR #794 parse implementation appears to cover this because it routes parsing through parseNat and only special-cases -, matching official std.jsonnet:

if str[0] == '-' then
  -parse_nat(str[1:], 10)
else
  parse_nat(str, 10)

So this is not a new issue if #794 lands, but it is worth keeping as an explicit direction test because go-jsonnet accepts std.parseInt("+1") while official Jsonnet and jrsonnet reject it.