Localizable error catalogue via a code-keyed message resolver

The 52 typed exceptions hard-coded their title/detail and the render invariant blocked any override, so the catalogue was neither overridable nor localizable despite being advertised as both. We make each error carry a context of placeholder values, turn title/detail into PSR-3 {placeholder} templates, and add an optional ErrorMessageResolver (title(code) / detail(code) → ?string template) that supplies a replacement template per stable error code.

Core is the sole interpolator — resolve the template (via the resolver, or the exception's inline default), then fill context — applied uniformly to every error in a response, so an integration binds only a thin adapter over its framework translator and locale negotiation stays the framework's job, not core's (the 422 title an integration emits localizes through the same seam for free). The two-method, copy-only interface means code/status are never overridable (they remain the machine and HTTP contract); an unfilled placeholder is left literal so the render path never throws; and with no resolver bound the output is byte-identical, which the existing error tests enforce. Chosen over injecting a translator into each exception, a central core message table, and integration-only override hooks.