fix(compiler): preserve literal variable reference text

[BichengWang]

What

Preserve literal @variables text when compiling expressions.

Why

Fixes #96. For example, @variables.x == "@variables.x" currently compiles to state.x == "state.x", changing the comparison. Literal text can also trigger an unknown-variable warning.

How

Remove the final regex replacement over serialized expressions. Reference AST nodes already resolve recursively, including inside function arguments. Add five regression cases covering parsed literals, comparisons, mixed literal/reference arguments, escaped quotes, and template text alongside real references.

Test Plan

All five new cases failed before the fix and pass afterward.

• Existing tests pass (pnpm exec turbo run test --force: 24/24 tasks; compiler: 1,118 passed, 3 skipped)
• New/updated tests cover the change
• Linting and type checks pass (pnpm exec turbo run lint --force and pnpm exec turbo run typecheck --force; lint reports 12 existing UI warnings)
• pnpm format:check, changed-file ESLint, and the server path traversal regression suite pass

Checklist

• My code follows the project's coding style
• I have reviewed my own diff
• I have added/updated documentation as needed (no documentation change needed)
• This change does not introduce new warnings