Use $ for value variables and reserve @ for scene key references

[TiernanDeFranco]

Motivation

• Standardize the scene format so that $ is used to declare and reference value variables while @ denotes scene key references like parents and keys.
• Update serialization and parsing logic to match the new token semantics and reflect the change across resource files and tests.

Description

• Add a Dollar token to the lexer and recognize $ as the prefix for value variables while keeping @ as a distinct token for key references.
• Update the parser to treat $<name> as value variables and @<name> as SceneValue::Key, and adjust parsing of tags, var collection, and header variable handling accordingly.
• Update the scene writer (SceneDocWriter) to emit $ for value vars and retain @ when writing parent/key references, and adjust dedupe variable emission to use $.
• Modify resource scene files and unit tests to use the new $ variable syntax and to use @ for explicit key references where appropriate.

Testing

• Ran the perro_scene unit test suite via cargo test -p perro_scene and all tests completed successfully.
• Verified that serialization round-trips by parsing written scene text back with Parser::new(...).parse_scene() in tests and assertions passed.

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Codex Task