feat(doctor): detect missing toolchain runtime deps / dangling subos symlinks

[Sunrisepeak]

Problem

When a provider xim package is removed, an already-installed toolchain is left in a silently-broken state: its compiler binary keeps an absolute RUNPATH baked in (and subos/default/lib holds symlinks) that now point at files which no longer exist.

The concrete case that motivated this: deleting xim-x-zlib left clang++'s RUNPATH pointing at the now-gone xim-x-zlib/<v>/lib. The package still compiled fine, but the produced binary died at runtime with:

libz.so.1: cannot open shared object file: No such file or directory   (exit 127)

Nothing in mcpp self doctor surfaced this — the breakage only showed up later as a mysterious build/link failure.

Fix

A new Checking toolchain runtime deps section in doctor_report(), Linux/ELF only (guarded by #if !defined(__APPLE__) && !defined(_WIN32); macOS/Windows skip it):

1. Enumerate installed toolchains the same way mcpp toolchain list does — iterate <xlingsHome>/data/xpkgs/xim-x-<compiler>/<version>/bin and resolve the frontend via toolchain::toolchain_frontend().
2. Read each compiler's RUNPATH/RPATH with readelf -d, run through the existing mcpp::platform::process::capture helper (no new process code, no new deps). Parse the Library runpath: [a:b:c] line, split on :. For each absolute entry whose directory is missing, warn(...) naming the toolchain + missing path, hinting the providing xim package may have been removed (reinstall the toolchain to repair).
3. Scan <xlingsHome>/subos/default/lib for dangling symlinks (std::filesystem::exists follows links → false when the target is gone) and warn(...) with the symlink + its broken target.

Exit-code semantics are unchanged — warnings only, no new errors.

The RUNPATH parsing is factored into an exported, process-free helper parse_readelf_runpath() so it can be unit-tested without spawning anything.

Testing

• mcpp build — passes (only the pre-existing imgui warnings).
• mcpp test — all 23 unit test binaries pass, including the new test_doctor_runpath (covers the zlib-removal RUNPATH shape, legacy DT_RPATH, no-runpath, and empty-token cases).
• mcpp self doctor (freshly built binary) — runs without crashing and prints the new section:

      Checking toolchain runtime deps
            ok all installed toolchain RUNPATH dirs present
            ok subos lib symlinks all resolve (/home/.../registry/subos/default/lib)
  

• Verified the dangling-symlink semantics with a fixture: a symlink to a removed xim-x-zlib/.../libz.so.1 reports as non-existent (follows the link), matching the detection branch.

Files changed

• src/doctor.cppm — new section + exported parse_readelf_runpath() helper + imports (mcpp.platform.process, mcpp.toolchain.registry).
• tests/unit/test_doctor_runpath.cpp — self-contained gtest coverage for the parser.