Template support for scroll usage

[dmtrshat]

This change adds template support for config-defined scrolls in g!…; syntax, including variable arguments and prefix modifiers, while keeping output deterministic and reducing unnecessary cloning.

Overview:

• Enabled using scrolls inside g!…;, e.g. g!complex-card=120px_red_100px;.
• Flattened scroll expansion into real property spells so CSS is generated under the outer templated selector.
• Preserved prefix semantics (md__, {...} focus, hover: effects) by applying them to each expanded spell.
• Reduced clone pressure in the template-flattening path.

Tests:

• Added unit coverage for parsing g!<scroll>=...;.
• Added end-to-end in-memory builder tests covering both plain and prefixed cases (md__{...}hover:).

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closes #99