fix: std.parseYaml YAML 1.2 octal (0o777) and document marker handling

[He-Pin]

Motivation

Two issues in std.parseYaml diverging from go-jsonnet:

1. YAML 1.2 octal: SnakeYAML's SafeConstructor uses YAML 1.1 type resolution which does not recognize the 0o prefix for octal integers (YAML 1.2), causing unquoted 0o777 to be parsed as string "0o777" instead of 511.
2. Document markers: Explicit document start markers (---) caused single-doc YAML to be returned directly instead of wrapped in an array as go-jsonnet does.

Modification

• JVM (src-jvm/Platform.scala): Replaced SafeConstructor-based parsing with composeAll() + custom yamlNodeToJson() that handles YAML 1.2 octal (0o prefix), hex (0x), binary (0b), sexagesimal, and special float values for plain scalars while preserving quoted values as strings. Added YamlDocStartPattern regex to detect explicit --- and wrap single-doc results in an array.
• JS (src-js/Platform.scala): Added Yaml12OctalPattern matching and isQuotedScalar position check to detect YAML 1.2 octal in the scala-yaml Node representation. Added same document marker detection.

Result

std.parseYaml now matches go-jsonnet for both YAML 1.2 octal syntax and document start marker handling.

YAML 1.2 Octal

YAML input	go-jsonnet v0.22.0	sjsonnet (before)	sjsonnet (after)
a: 0777	511	511	511
b: 0o777	511	"0o777" (bug)	511
d: 0o10	8	"0o10" (bug)	8
e: -0o777	-511	"-0o777" (bug)	-511
f: "0o777"	"0o777"	"0o777"	"0o777"

Document Markers

YAML input	go-jsonnet v0.22.0	sjsonnet (before)	sjsonnet (after)
"---"	[null]	null (bug)	[null]
"---\na: 1"	[{a:1}]	{a:1} (bug)	[{a:1}]
"a: 1"	{a:1}	{a:1}	{a:1}

Test plan

• All ParseYaml tests pass (14 tests)
• All FileTests pass (including parseyaml_yaml12_octal.jsonnet and parseyaml_doc_marker.jsonnet)
• All EvaluatorTests pass
• All go_test_suite golden tests pass
• Code passes scalafmt
• Verified against go-jsonnet v0.22.0 and jrsonnet v0.5.0-pre99