feat: Add CacheResolver for fixed-precedence cache routing

[jzombie]

Added

• CacheResolver + CacheSource: combined cache-root resolution with fixed
  precedence (explicit path → env var → Cargo workspace discovery → OS user
  cache) and no silent <cwd>/.cache scattering. Unresolvable configurations
  return Err(NotFound) naming the env var / OS identity that would fix it;
  the <cwd>/.cache last resort is an explicit allow_cwd_fallback opt-in.
  The winning source is always returned so binaries can announce cache
  placement. Includes resolver unit tests (precedence matrix, NotFound
  defaults, panic-safe env guards) and a README section documenting the
  contract and the announce-the-winner convention.

Fixed

• README doctests for from_project_dirs failed under default features
  (directories unavailable, method compiled out). Examples are now
  feature-gated run() functions: fully compiled and executed wherever the
  os-cache-dir feature is enabled, never ignored, never stale.