Replace logos-based RDL formatter with proc_macro2 TokenTree traversal

[Copilot]

Reattempts #4009: replaces the hand-written logos-based RDL formatter with one that traverses proc_macro2's native token tree directly. Only two files change.

Changes

• formatter/mod.rs — Removed ~370 lines (logos enum, manual depth counters, bespoke push_attribute char-scan). Replaced with ~185 lines of TokenTree recursion:

  • { }, ( ), [ ] are already Group nodes in proc_macro2 — no manual depth counters needed; the formatter recurses at the correct indentation level automatically
  • < > generics still use an explicit angle_depth: usize counter (not grouped by the tokenizer)
  • :: and -> detected via Spacing::Joint rather than dedicated lexer tokens
  • ... variadic: joint dots skip the space-trim so , ... formats correctly
  • #[...] attributes parsed recursively in inline mode, eliminating the custom char-scanner

• Cargo.toml — Removed logos = "0.16". proc_macro2, syn, and quote were already workspace dependencies.

All existing test fixtures produce identical output.