feat(toolchain): dimensional ABI compatibility model (fix glibc C-lib under libc++)

[Sunrisepeak]

Problem

The capability-driven ABI check (added in 0.0.54) derives a single tcAbi string where stdlibId == "libc++" short-circuits before the glibc fallback:

tcAbi = triple.find("musl") ? "musl"
      : stdlibId == "libc++" ? "libc++"   // ← conflation
      : compiler == MSVC     ? "msvc" : "glibc";

So a clang+libc++ toolchain on x86_64-unknown-linux-gnu — which is glibc at the libc level — is labelled abi=libc++ and rejects any dependency declaring abi:glibc, even pure C libraries like glfw (libc++ is a C++ stdlib, not a libc replacement; it links glibc fine). This blocks building glfw, and anything pulling it, under the llvm toolchain (e.g. the imgui-m module package, which pins llvm@20.1.7).

Full root-cause + reproduction: .agents/docs/2026-06-27-glfw-abi-glibc-vs-libcxx-conflation-analysis.md.

Fix — model ABI as the orthogonal axes it is

dimension	values	source
libc	glibc / musl / msvcrt / macos	target triple
cxxStdlib	libstdc++ / libc++ / msvc-stl	compiler / stdlibId
arch, os	…	target triple
cxxAbi	itanium / msvc	compiler

A dependency constrains only the dimension(s) it cares about; unspecified dimensions are don't-care. Legacy abi:glibc → the libc dimension only (zero descriptor changes); a new abi:<dim>=<val> form expresses other axes.

Design (single-PR scope, with the L3 reselection seam): .agents/docs/2026-06-27-abi-compat-model-single-pr-design.md.

Changes

• new src/toolchain/abi.cppm — AbiProfile + abi_profile() (single derivation site), parse_abi_capability(), abi_check() (per-dimension, don't-care).
• src/build/prepare.cppm — replace the conflated tcAbi block with the dimensional check; precise per-dimension diagnostics.
• src/doctor.cppm — show abi(libc)/cxxstdlib/arch/os instead of one fused string.
• tests/unit/test_abi.cpp — 11 cases incl. the regression lock.
• version → 0.0.68 (mcpp.toml + fingerprint.cppm).

Verification (local, fresh 0.0.68)

• glfw under gcc@16.1.0 → Finished (no regression)
• glfw under llvm@20.1.7 → Finished ✅ (the fix)
• glfw under --target x86_64-linux-musl → still errors: requires libc=glibc … provides libc=musl (genuine incompatibility, now naming the dimension)
• mcpp test: 27 unit binaries pass incl. new test_abi
• doctor: abi(libc)=glibc cxxstdlib=libc++ arch=x86_64 os=linux

Once released, mcpp-index can move the glfw/imgui-module GL smokes back into the blocking CI set.