Add opt-in memoized left-recursion support for marked rules

[Copilot]

This adds an opt-in path for PEG rules that need memoization-backed left recursion. Marked rules can now grow through direct or mutual left-recursive expansions without rewriting the grammar into a non-left-recursive form.

• Grammar syntax

  • Add a recursive rule modifier after =
  • Support @ as the documented marker, with % kept as an equivalent alias
  • Parse recursive and silent modifiers independently so they can be combined

• Meta parser plumbing

  • Extend ParseRule with a recursive flag
  • Thread the flag through grammar parsing and error naming
  • Keep unmarked rules on the existing behavior path

• Generated parser behavior

  • Emit recursive-aware wrappers only for rules explicitly marked recursive
  • Leave non-recursive generated rules unchanged to avoid widening trait bounds or altering existing parse behavior

• Runtime memoization

  • Add per-rule/per-position memoization in the typed tracker for marked recursive rules
  • Use iterative growth to reuse the best known result and stop when no further input is consumed
  • Apply the same strategy to both full parse and check-only paths

• Coverage and docs

  • Add focused tests for direct and mutual left recursion
  • Document the new marker syntax in derive docs

Example:

expr = @ { expr - "+" - num | num }
num = ('0'..'9')+

call    = @ { primary - args }
primary = @ { call | ident }
ident   = ('a'..'z')+
args    = "(" - ")"

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