feat: make C++ standard a first-class build setting

[Sunrisepeak]

Motivation

[package].standard is parsed today, but C++ compile flags and import std BMI prebuilds still have C++23 hardcoded in key places. Users can currently work around C++26 by adding -std= in build.cxxflags, but that bypasses the intended manifest/fingerprint/std-module model and can leave cached std BMIs built with the wrong dialect.

Scope

• Treat the active C++ standard as a first-class build setting, defaulting to C++23.
• Support C++26 spellings such as c++26 and c++2c, with room for GNU dialect aliases.
• Ensure build flags, fingerprinting, compile_commands, and std module BMI prebuilds consume the same active standard.
• Add metadata validation for cached std module BMIs so import std cannot reuse a BMI built with a different C++ standard.
• Keep build.cxxflags as additional C++ flags, not the language-standard configuration path.
• Update docs and regression coverage.

Design Notes

The design draft is tracked in .agents/docs/2026-06-01-cpp-standard-first-class-design.md.

Test Plan

• Unit tests for manifest standard parsing and aliases.
• Unit tests for generated build flags and std module build commands.
• E2E coverage for C++26 flags in build.ninja and compile_commands.
• Toolchain-gated import-std checks where supported by the CI environment.