Motivation:
Two issues in std.parseYaml diverging from go-jsonnet:
1. 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. Explicit document start markers (---) caused single-doc YAML to be
   returned directly instead of wrapped in an array as go-jsonnet does.

Modification:
Replaced SafeConstructor-based parsing with composeAll() + custom
yamlNodeToJson() that handles YAML 1.2 octal (0o prefix) for plain
scalars while preserving quoted values as strings. Added
YamlDocStartPattern regex to detect explicit --- and wrap single-doc
results in an array.

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

| YAML input | go-jsonnet v0.22.0 | jrsonnet 0.5.0-pre99 | sjsonnet (before) | sjsonnet (after) |
|-----------|-------------------|---------------------|-------------------|-----------------|
| 0o777     | 511               | 511                 | "0o777" (bug)     | 511             |
| -0o777    | -511              | -511                | "-0o777" (bug)    | -511            |
| "0o777"   | "0o777"           | "0o777"             | "0o777"           | "0o777"         |
| "---"     | [null]            | null                | null (bug)        | [null]          |
| "---\na:1"| [{a:1}]           | {a:1}               | {a:1} (bug)       | [{a:1}]         |
| "a: 1"    | {a:1}             | {a:1}               | {a:1}             | {a:1}           |

Note: jrsonnet does NOT wrap --- in array; sjsonnet aligns with go-jsonnet.