Refactor CommandArg.TryGet type dispatch into a table-driven registry

[wallstop]

DISCLOSURE: LLM-GENERATED TEXT

Follow-up from PR #57 review (CommandArg.cs:209): CommandArg.TryGet<T> is one giant
method — a ~400-line if (type == typeof(X)) chain with inline switch cases per
composite type, plus local helper functions. It works (192/192) but is hard to navigate,
and every new type widens the chain.

Proposed shape

• Table-driven dispatch: a static Type -> parser delegate map built once and consulted
  by TryGet (the RegisteredParsers/StaticProperties/ConstFields/EnumValues
  caches already establish the pattern).
• Composite-type parsing (Vector2/3/4, Vector2Int/3Int, Color, Quaternion, Rect,
  RectInt) moves to per-type parse functions beside the new public
  CommandArgParsers class, so delimiter/quote handling stays in one place.
• Keep public behavior exactly: same accepted formats, same TryGet semantics, same
  precedence (per-call parser override -> registered parser -> built-in -> enum ->
  named constants). The culture-invariance work in Warnings as errors + built-in analyzers (#55), user-facing changelog (#54) #57 (NumberStyles + InvariantCulture
  pinned per type) must survive unchanged, along with the ParsingIsCultureInvariant
  PlayMode cases.
• No public API change: TryGet, TryGet<T>(parser), RegisterParser,
  CommandArgParser<T> keep their signatures.

Acceptance

• Existing PlayMode suites pass unchanged (TryEatArgument parity,
  ParsingIsCultureInvariant, custom-parser cases).
• Adding a built-in type = one table entry + one parser function, not another
  else if arm (pairs with the built-in-types issue).
• No allocation regressions on the TryGet hot path (parse calls stay direct delegates;
  no boxing in the common path).