feat(build): make C++ standard first-class

[Sunrisepeak]

Closes #98

Summary

• Add normalized [package].standard support for c++26, c++2c, gnu++23, gnu++26, and legacy [language].standard compatibility.
• Route the active C++ standard through fingerprinting, compile flags, compile_commands.json, P1689 scanning, and import std BMI builds.
• Add std module cache metadata so cached std.gcm / std.pcm is reused only when compiler, stdlib, target, standard, source, and build commands match.
• Keep build.cxxflags as package-owned additive flags and reject -std= there with a migration hint.
• Reuse persistent CI toolchain payloads from temp-home e2e tests and prewarm musl-gcc once in Linux CI, avoiding repeated 808 MB musl downloads.
• Fix transitive dependency include propagation so each consumer package receives its dependency public include dirs.
• Document [package].standard, the C++ standard architecture, and the CI toolchain-cache analysis; bump version to 0.0.42.

Verification

• mcpp test -- --gtest_brief=1 passed.
• tests/e2e/59_cpp_standard_config.sh passed, including the P1689 scanner path.
• tests/e2e/29_toolchain_partial_versions.sh passed.
• tests/e2e/31_transitive_deps.sh passed.
• bash -n tests/e2e/_inherit_toolchain.sh tests/e2e/29_toolchain_partial_versions.sh tests/e2e/31_transitive_deps.sh passed.
• git diff --check passed.

[Sunrisepeak]

CI is green on Linux, Windows, and macOS.

Merge is currently blocked only by the required-review policy (reviewDecision=REVIEW_REQUIRED). I requested maintainer review so this can proceed through the normal gate; I am not using admin override.