Remove Option Wrapper from Rule Precedence

[ehwan]

Description

This PR moves the “no precedence” state into core::rule::Precedence itself by adding Precedence::None, instead of representing rule precedence as Option<Precedence>.

Changes

• Added Precedence::None and made it the default variant.
• Changed production rule and parser table precedence fields from Option<Precedence> to Precedence.
• Updated builder, deterministic parser, and nondeterministic parser logic to match on Precedence::None.
• Updated parser code generation to serialize and decode Precedence::None.
• Updated checked-in generated parser output and diff snapshots.

[gemini-code-assist]

Code Review

This pull request refactors the Precedence representation by replacing Option<Precedence> with a direct Precedence enum that now includes a None variant. This change simplifies the codebase and eliminates unnecessary option wrapping across grammar building, parsing, and code emission modules. There are no review comments, so I have no feedback to provide.

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