feat: improve type-test error messages to be actionable

[martyy-code]

Description

When a type test fails, the TypeScript compiler error is cryptic (e.g. Type 'string | number' is not assignable to type '"a" | "b"'). Add a translation layer that renders actionable error messages: which key is missing, which type is wrong, what the expected shape was.

Why

Cryptic errors are the #1 reason developers give up on type testing. If a failing test produces a readable error, developers can fix it without deep TS knowledge. This is a DX multiplier.

Acceptance criteria

• Error messages show expected vs received type in plain terms
• For array/object mismatches: show the index/key that diverges
• For union mismatches: show which union member was expected
• Configurable verbosity (terse / detailed)

Risks

• Low. TypeScript's error API is stable. Start with string/regex rewriting.

Related

• feat: implement IsInhabited as a first-class type primitive #49 (IsInhabited)
• expectTypeOf(A).toEqualTypeOf<B>() does not cause Vitest to fail tests mmkal/expect-type#148