feat(solutions+ci): add reference solutions and dslings build CI for cpp11

[Sunrisepeak]

Summary

Adds a solutions/ mirror of dslings/ containing canonical, fully-compilable reference answers for every cpp11 exercise, and a CI workflow that builds + runs them through the same xmake d2x-buildtools plugin path that d2x checker uses internally.

Why

Practice files under dslings/ are intentionally broken (D2X_YOUR_ANSWER placeholders + deliberate compile/runtime bugs), so they can't be used directly for build CI. Reference solutions give CI a known-passing endpoint per exercise:

• Any drift in xmake target / include paths / language standard surfaces as a build failure (this is what bit us in 练习: 08-literal-type-0 英文(en) 没有设置C++17 导致报错 - non-constexpr function 'operator[]' cannot be used in a constant expression #37 — dslings/en/cpp11/xmake.lua had silently lost set_languages("c++17") for cpp11-08-literal-type-0).
• Any teaching mistake (the canonical answer can't actually pass its own d2x_assert checks) surfaces as a runtime failure.
• Future zh ↔ en drift gets flagged early.

What's in this PR

• solutions/cpp11/<NN>-<feat>-<K>.cpp — 47 reference answers, one per practice file. Each is the practice file with all D2X_YOUR_ANSWER filled in, all intentional bugs fixed, and D2X_WAIT removed. All D2X_DONT_DELETE_THIS(x) calls and d2x_assert* checks are preserved byte-for-byte from the practice (no expected-value tampering).

• solutions/cpp11/xmake.lua — mirrors dslings/cpp11/xmake.lua. Each target carries a -ref suffix so it coexists with the practice target (e.g. cpp11-13-long-long-0 and cpp11-13-long-long-0-ref). Per-target flag overrides (-fno-elide-constructors, -Wpedantic -Werror, set_languages("c++17")) are mirrored.

• solutions/xmake.lua — top-level loader. Only includes cpp11 today; cpp14/17/20/23 will follow as those chapters get written.

• xmake.lua — one-line addition: includes("solutions/xmake.lua").

• .github/workflows/dslings-ref-ci.yml — runs on push/PR/dispatch when dslings/, solutions/, d2x/, or xmake.lua change. Installs pinned xlings 0.4.3 + xmake, then:

  1. Lists every *-ref target via xmake d2x-buildtools --command=list
  2. Builds each via xmake d2x-buildtools --command=build
  3. Runs each and asserts no ❌ and no stray D2X_WAIT
  4. Lints that every dslings/*.cpp has a paired solutions/*.cpp (warning during bring-up; can tighten to error later)

  The workflow uses the same plugin entry point (d2x/buildtools/xmake/main.lua) that d2x checker uses internally, so a green CI here is exactly what a learner would see if they ran d2x checker cpp11-XX-feat-K-ref on every solution locally.

Test plan

Local clean-cache sweep through all 47 reference targets via the d2x-buildtools path:

• xmake clean -a && xmake f -y succeeds
• All 47 *-ref targets build successfully
• All 47 produce only ✅ at runtime — 0 ❌, 0 stray D2X_WAIT
• Existing dslings/ practice files still produce their intentional teaching errors (didn't break the student experience)
• Asserts preserved byte-identical: spot-checked 5 trap exercises (08, 10, 11, 17, 05), only differences are added explanatory comments
• Negative tests pass: a deliberately-wrong solution produces ❌ and CI exits 1; a stray D2X_WAIT triggers the residue check and CI exits 1
• YAML syntax of the new workflow is valid (yaml.safe_load)

Trap exercises — how each was solved

• 08-literal-type-0: std::string is not a literal type in C++17. Removed constexpr from the std::string chain; kept it on the char/int/std::array chain and called to_array("123") directly so the constexpr arm still demonstrates compile-time computation.
• 10-delegating-constructors-0: 1-arg → 2-arg → 3-arg delegation. Counter increments 0→3→5→6 to match the existing d2x_assert_eq values.
• 11-inherited-constructors-2: deliberately did not declare move ops, so std::move(p3) falls back to copy — that's the teaching point.
• 17-pod-type-0: ok = !std::is_trivial<C>::value — C has a virtual function so it's not trivial; the assert wants ok == true.
• 15-variadic-templates-0/1: standard C++11 recursive base case + variadic recursion.
• 05-move-semantics-0: copy ctor transfers via const_cast so all three == asserts on data_ptr() hold and there's exactly one alloc/free.
• 06-scoped-enums-0: Color::ORANGE renamed to ORANGE_COLOR to dodge the unscoped/scoped collision.

Notes & follow-ups

• cpp14/17/20/23 to follow when those chapters are added.
• The solutions ↔ dslings filename-parity step is a soft warning during bring-up. After cpp11 stays at 100% coverage for a release cycle, flip it to an error so missing solutions block merges.
• This is the second consumer of pinned xlings 0.4.3 (after ci: pin xlings to 0.4.3 and simplify install step #43's online-ebook update). If we end up with 3+ workflows installing xlings, worth extracting into a composite action.