Remove lookahead pattern syntax and forced rule lookaheads

[ehwan]

Removes the P / term lookahead pattern syntax and the associated forced lookahead plumbing.

Changes

• Removed PatternArgs::Lookaheads and PatternInternal::Lookaheads
• Removed Pattern slash Pattern parsing support and regenerated the bootstrap parser
• Removed rule-level forced lookaheads from rusty_lr_core builder logic
• Removed the NonTerminalType::Lookahead marker
• Updated syntax/docs references

[gemini-code-assist]

Code Review

This pull request removes the lookahead assertion feature from the parser generator. This cleanup simplifies the grammar, parser, and pattern matching logic by removing lookahead-related fields, types, and parsing rules across multiple files, including rusty_lr_core, rusty_lr_parser, and associated documentation. I have no additional feedback to provide as the changes are clean and consistent.

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