v1.0.3

Patch release closing the three actionable findings from
docs/assessment-maintainable-architect-v4-v1.0.2.md, which reconciled
two disagreeing Codex reviews of v1.0.2 (one reporting zero findings,
one reporting five) through independent adversarial verification. No
breaking changes; module path unchanged.

Fixed:

• A typed-nil coded error (e.g. a *NotFoundError returned nil on some
  path, then assigned to a plain error) panicked response rendering
  instead of classifying as ErrCodeInternal. errors.As matches on
  type, not nilness, so the classic Go typed-nil-interface footgun
  reached Code() on a nil receiver before any accessor ran.
  outermostCoded now rejects a typed-nil match centrally, so
  GetErrorCode and every JSON/problem body writer that shares it
  degrade to ErrCodeInternal instead of dereferencing.
• An external Coder-only implementation (a documented, first-class
  extension point that doesn't require embedding BaseError) could
  return "" from Code() and have it ride straight through to the
  wire as the machine-readable error code. New/Wrap already
  normalized this for the package's own constructors as of v1.0.2;
  GetErrorCode now normalizes it too, closing the gap for bare
  third-party types at the one extraction point every renderer funnels
  through.

Documented:

• Coder, ProblemTyper, ProblemInstancer, ProblemTitler, and
  Authenticator now state their contract explicitly: a true bool
  return implies a non-empty string, the same guarantee BaseError's
  own implementations already make. No code change - a third-party
  implementation following the documented convention was already safe.
• Two design choices raised by the v1.0.2 review and confirmed
  deliberate rather than oversights are now recorded as ADRs instead of
  living only in review prose: Logger has no context.Context
  parameter (docs/adr/0001),
  and a marshal-panic log keeps the original error's stack rather than
  capturing a second one at the panic site
  (docs/adr/0002).

Test coverage: 100.0% (root module), 100.0% (zerologadapter, unchanged
this release).