feat(core): std::expected error handling #8

Goal

A common error type and helpers for fallible functions.

Scope

• Error (code or category, message, optional source location), Result<T> = std::expected<T, Error>, Status = Result<void>, small helpers (makeError, and a propagation macro such as AETHER_TRY only if justified).
• Pure std::expected (C++23), no exceptions.

How to verify

• Tests for success, failure, propagation and Result<void>.
• Static and shared presets; clang-tidy clean.

Dependencies

#5 (independent of #6 and #7)

Suggested model

sonnet