Extend built-in parseable argument types (Unity: Bounds, BoundsInt, RectOffset, Plane, Ray; System: Complex)

[wallstop]

DISCLOSURE: LLM-GENERATED TEXT

Follow-up from PR #57 review (CommandArg.cs:310): are there other built-in types worth
supporting natively in CommandArg.TryGet<T>, given the console's delimiter and quoting
model?

Current built-in coverage

• Primitives: bool, char, byte/sbyte, short/ushort, int/uint, long/ulong,
  float/double/decimal, BigInteger, string.
• System: Guid, Version, DateTime, DateTimeOffset, TimeSpan, IPAddress.
• Enums (name or numeric index) plus named-constant lookup for static
  properties/const fields (red, MaxValue).
• Unity math types: Vector2/3/4, Vector2Int/3Int, Color, Quaternion, Rect, RectInt.

Candidates that parse naturally from delimited text

• Unity: Bounds (center/size), BoundsInt (position/size), RectOffset
  (left/top/right/bottom), Plane (normal + distance), Ray (origin + direction).
• System.Numerics: Complex (re, im) — BigInteger is already supported, so this is
  consistent.
• Possibly Matrix4x4 and Vector4-based color formats — heavier, decide separately.

Caveats to settle before implementing

1. Text format per type must be documented in the command help surface and survive the
   shell's delimiter set (,, ;, :, _, /, \) — e.g. Bounds uses : inside
   Unity's own ToString(), which collides with the delimiter set; choose an
   unambiguous format or accept Unity-ToString-style input like the Rect parser does
   (x:/width: stripping).
2. Each addition rides the culture-invariant path from Warnings as errors + built-in analyzers (#55), user-facing changelog (#54) #57 (CommandArgParsers, pinned
   NumberStyles + InvariantCulture).
3. Cheap types first; anything needing allocation-heavy parsing (AnimationCurve,
   Gradient) is likely out of scope for a console argument.
4. Data-driven PlayMode coverage per type in CommandArgTests, matching the house
   style, plus ParsingIsCultureInvariant cases where relevant.

Related: the TryGet dispatch refactor issue (table-driven registry) should land first or
together, so each new type is one table entry, not another else if arm.