Remove %err alias and demote advanced parser controls

[ehwan]

Summary

• Remove %err as an alias for %error in the grammar lexer
• Update examples to use %error
• Move box, custom token ranges, and dynamic %prec documentation into the advanced parser controls section
• Simplify README generated-code guidance by keeping internal generated types out of the main flow
• Move context inspection helpers and detailed CLI diagnostic options into debugging/advanced sections

[gemini-code-assist]

Code Review

This pull request standardizes the error type directive by replacing %err and %errortype with %error across the parser implementation, examples, and documentation. It also reorganizes the documentation in README.md and SYNTAX.md to group advanced features—such as memory optimization with box, custom token ranges, and dynamic precedence—into dedicated sections, and updates the CLI usage guide in rusty_lr_executable/README.md. Feedback is provided to update the supported variables section in SYNTAX.md to remove references to %errortype for consistency.

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[gemini-code-assist]

Since %errortype is no longer supported as a directive (and has been replaced by %error), please also update the description of $error or $errortype in the Supported Variables section below (around line 509) to remove the reference to %errortype for consistency.