feat: add mcpp build support and switch Linux CI to mcpp

Replaces the existing CI with a single mcpp-driven Linux job:
- `.xlings.json` declares `mcpp = { linux = "0.0.3" }`.
- `.github/workflows/ci.yml` boots xlings, runs `xlings install -y`,
  then `mcpp build` + `mcpp test` against the existing mcpp.toml.

mcpp 0.0.3's binary lives at xim-x-mcpp/0.0.3/bin/mcpp; on first
run it self-detects home as that grandparent and bootstraps musl-gcc
+ binutils + glibc + ninja + patchelf into <home>/registry/data/xpkgs.
Toolchain set is version-pinned by mcpp 0.0.3, so a fixed cache key
is safe. Cache-hit runs skip the ~800 MB download.

* chore: migrate deps to namespace syntax (mcpp 0.0.6)

Updates mcpp.toml to use explicit namespace fields:
- [package].namespace added
- [package].name uses short name only
- [dependencies.compat] / [dev-dependencies.compat] for non-modular libs
- [dependencies.mcpplibs.capi] for C API wrappers

* ci: retrigger with fixed mcpp 0.0.6

* ci: retrigger

* ci: retrigger with mcpp 0.0.7

gtest is a compat (non-modular) package in mcpp-index. Declare it
under [dev-dependencies.compat] so mcpp resolves the correct namespace.

Required for mcpp 0.0.10+ namespace-aware dependency pipeline.

- .xlings.json: mcpp 0.0.7 → 0.0.11
- CI: xlings 0.4.25 → 0.4.30, mcpp 0.0.11
- README: expand mcpp build/usage instructions

* feat: add shell completions skeleton

* feat(completions): add fish support

* docs: add shell completions example

* build: re-support xmake

* feat(completions): add bash support

* feat(completions): add zsh support

* docs: update api.md with shell completions

* test(completions): add shell completion tests

* fix(completions): fix linkage for gcc 15.1.0-musl

Move SubCmdEntry and ScBranch from anonymous namespace to named `detail`
namespace in bash and zsh completion partitions.

GCC 15.1.0 enforces C++20 module linkage rules more strictly: types with
internal linkage (anonymous namespace) cannot appear as template
arguments to module-attached templates like std::vector. This exposes
TU-local entities through template instantiation, causing "exposes
TU-local entity" errors.

ScBranch additionally required moving to namespace scope because local
structs (no linkage) triggered an ICE when instantiating std::vector
inside an external-linkage function in a module implementation.

* chore: bump mcpp 0.0.11 to 0.0.89