fix: std.parseYaml supports YAML merge keys (<<)

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fix: std.parseYaml supports YAML merge keys (<<)

Motivation

std.parseYaml did not support YAML merge key expansion. The << key was kept as a literal string key instead of merging the referenced mapping's fields into the parent. Both go-jsonnet and jrsonnet correctly implement this YAML spec feature.

Example:

defaults: &def
  x: 1
  y: 2
item:
  <<: *def
  z: 3

Expected: {"defaults":{"x":1,"y":2},"item":{"x":1,"y":2,"z":3}}
Before: {"defaults":{"x":1,"y":2},"item":{"<<":{"x":1,"y":2},"z":3}}

Modification

• Platform.scala (JVM): Modified the MappingNode handler in yamlNodeToJson to detect Tag.MERGE on key nodes and inline the referenced mapping's entries with lower priority than explicit keys (per YAML spec). Supports both single alias (<<: *def) and sequence of aliases (<<: [*a, *b]).
• Test skipped on JS/WASM/Native (different YAML parser — scala-yaml).
• Added regression test: parseyaml_merge_keys.jsonnet

Result

Merge keys are now correctly expanded, matching go-jsonnet and jrsonnet output.

Implementation	Behavior
go-jsonnet	Merge keys expanded ✅
jrsonnet	Merge keys expanded ✅
C++ jsonnet	Anchors not supported (N/A)
sjsonnet (before)	<< as literal key ❌
sjsonnet (after)	Merge keys expanded ✅