Introduce $ sigil for value variables and reserve @ for node refs; update parser, writer, lexer, docs and templates

[TiernanDeFranco]

Motivation

• Remove ambiguity between scene-level value variables and node references by introducing distinct sigils: $ for value vars and @ for node refs/scene keys.
• Update the scene parser, lexer, and scene writer so the format is unambiguous and consistent across code and docs.
• Refresh authoring templates and example .scn/animation docs to use the new sigil conventions.

Description

• Add a Dollar token to the lexer and update the parser to treat $ as the value-var sigil and @ as node reference, including new parsing branches in parser.rs and variable collection changes in collect_var_entries and parse_value.
• Update scene document writer (scene_doc.rs) to emit $ for value vars and @ for node refs, and adjust dedupe/var output accordingly.
• Change error messages and behavior in scene preparation (prepare/core.rs) to reference $root where applicable when a missing root is detected.
• Revise many documentation files and scene/templates (docs/*, perro_editor/res/*.scn, perro_editor/res/proj_manager.scn, scene node templates) to reflect the new sigils and examples for AnimationPlayer/AnimationTree bindings and scene parent/key usages.

Testing

• Ran unit tests for the scene crate with cargo test -p perro_scene which includes parser and scene_doc tests, and they passed after updating test expectations to the new syntax.
• Ran scene loader unit tests in the runtime crate (tests touching root_of and static boot scenes) via cargo test -p perro_runtime and they passed with the updated input/expectations.
• Ran cargo test for the modified workspace crates to verify integration; all affected tests succeeded.

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Codex Task