Migrate grammar DSL to operator-only Type‑0 model and add rewrite-rule operators

[Copilot]

This updates the internal grammar surface to a Type‑0 rewrite-oriented model: Rule variables + terminal literals + first-declared start rule + operator syntax for productions/rewrite rules. It removes legacy builder/chaining APIs from grammar declaration paths and introduces explicit Empty semantics for ε.

• Core grammar model (Type‑0)

  • Simplified Rule composition to operator-only grammar construction (+, |).
  • Added Rule.Empty as the canonical ε symbol.
  • Start symbol selection now uses the first declared Rule field in declaration order.

• Terminal model simplification

  • Replaced TokenSpec hierarchy with a minimal terminal-oriented Token wrapper.
  • Added direct terminal conversions for grammar declarations: char, byte, int (hex-byte range), and byte[].

• Rewrite system syntax

  • Added rewrite primitives:
    • RewriteProduction (α -> β)
    • RewriteRule (collection of productions)
  • Enabled operator syntax:
    • (alpha) > (beta) to create rewrite productions
    • | to compose multiple productions into one RewriteRule.

• API cleanup / syntax migration

  • Removed legacy builder-style grammar APIs from the public DSL path (Ref, optional/repetition/list/precedence builder helpers, and chained rewrite methods).
  • Updated in-repo grammar declarations to operator-only forms (no .Ref, .Then, .Or, .To, .Rewrite, .Transform).

• Usage shape (new DSL)

  public static readonly Rule Expr =
      Number | (Expr + Plus + Expr);
  
  public static readonly RewriteRule R1 =
      (A + B + 'a' + C) > (D + 'b' + 'b' + A)
    | (A + 'x') > (A + 'y')
    | (C + 'z') > Rule.Empty;