fix: harden Result state representation

[zuudevs]

Summary

Starts roadmap milestone v0.8 by hardening the project-owned Result<T> contract without replacing the current std::variant storage baseline.

Result changes

• switch state inspection from type-based lookup to variant index/discriminant
• switch accessors to index-based std::get_if so wrong-state access does not hide a std::bad_variant_access path inside noexcept
• retain the documented precondition model for wrong-state access
• add explicit Result<T>::success(...) and Result<T>::failure(...) factories
• make Result<Error> capable of representing both success and failure unambiguously
• direct Result<Error>{Error{...}} remains the failure form; successful Error payloads use Result<Error>::success(...)
• preserve move-only payload support
• explicitly reject Result<void> until a concrete specialization is required
• keep std::variant rather than adding custom manual storage before measurement justifies it

Tests

Adds dedicated Result<T> coverage for:

• success and failure states
• mutable/const value access
• convertible success inputs
• move-only payloads
• copy/move trait behavior
• explicit factories
• Result<Error> success/failure distinction

Documentation

• updates the Result<T> contract with explicit factory semantics and the Result<Error> rule
• synchronizes docs/roadmap.md: v0.5, v0.6 and v0.7 are complete; v0.8 is now in progress
• records URL/query correctness as the next v0.8 substep

Scope

This PR does not change HTTP, transport, redirect, or connection-reuse behavior. It is the first focused correctness-hardening PR for v0.8.

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Preserve braced success construction

When a success payload is supplied as a braced initializer, such as Result<std::vector<int>>{{1, 2, 3}} or return {{1, 2, 3}}, template argument deduction cannot infer U, so code accepted by the previous Result(T) constructor now fails to compile. Preserve a non-template path for constructing T from a braced initializer while retaining the special handling required for Result<Error>.

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