Use flat parser tables and initialize them through Context

[ehwan]

Summary

• Replace generated get_rules() / get_states() access with a single Parser::get_tables() API.
• Store a decoded &'static P::Tables reference inside deterministic and GLR contexts during Context::new.
• Add dense and sparse flat table layouts that preserve the existing emit_dense layout decision.
• Merge shift and reduce terminal lookups into a single terminal action table.
• Split hot-path rule metadata into compact RuleInfo.
• Rename generated runtime table aliases from <Start>State to <Start>Tables.
• Add comments explaining the flat table layouts, merged actions, and context-held table references.

Motivation

Parsing repeatedly queries the same (state, terminal_class) pair for both shift and reduce behavior. The previous structure kept state-local maps and separate rule/state accessors, which added extra lookups and indirection in the hot path.

This change keeps the compact generated encode/decode style, but decodes into runtime tables optimized for parsing.

[gemini-code-assist]

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