feat(build): Windows runtime-DLL deployment beside the executable (v0.0.73)

[Sunrisepeak]

Summary

A directly-launched Windows .exe cannot RUNPATH-locate a dependency's runtime DLL — PE has no rpath mechanism, so mcpp's existing [runtime].library_dirs → RPATH path (ELF/Mach-O) is a no-op on Windows. This PR adds the symmetric PE backend: each *.dll found in a linked dependency's [runtime] library_dirs is staged into bin/ beside the produced executable via a ninja copy edge that the executable target takes as an implicit dependency.

This completes an abstraction already designed but unimplemented on PE:

• ELF / Mach-O → -Wl,-rpath,<dir> (unchanged)
• PE → copy *.dll beside the .exe (new)

Both are "make the runtime library locatable." It unblocks a Windows compat.openblas (import-lib + runtime DLL) and any future prebuilt-DLL package (fftw / hdf5 / …).

Design choice: *.dll-glob filter, not a platform if constexpr, no schema change

The deploy list is populated by globbing *.dll in each dependency runtime dir — not gated by if constexpr(is_windows) and with no manifest.cppm schema change:

• A real Linux/macOS dependency ships .so/.dylib (never .dll), so the glob matches nothing → non-Windows builds are byte-for-byte unchanged.
• A recipe declares [runtime] library_dirs globally; only a Windows prebuilt-DLL package populates the deploy list. (This is the design's §7 "declare it globally — harmless on Linux" option, made safe by the extension filter.)
• The same code path is therefore testable on a Linux host, no Windows runner needed for the mechanism.

Changes

• src/build/plan.cppm — BuildPlan::runtimeDeployFiles: glob *.dll from each dep [runtime] library_dirs, dedup by dest (bin/<dll>).
• src/build/ninja_backend.cppm — emit one cp_bmi copy edge per staged DLL; executables (Binary/TestBinary) implicitly depend on them; added to default. (Ninja auto-creates the bin/ parent dir before the edge runs.)
• tests/e2e/84_runtime_dll_deploy.sh — new e2e: a dummy dependency ships a stub libdummy.dll; asserts mcpp stages it beside the exe byte-for-byte and does not deploy a non-DLL sibling.
• Version bump 0.0.72 → 0.0.73 (mcpp.toml + fingerprint.cppm).
• Design doc updated with the implementation note / deviation.

Verification

• New e2e 84_runtime_dll_deploy.sh — PASS (Linux host, exercises the exact Windows copy path).
• Unit + integration suite (mcpp test) — 27 ok, 0 failed.
• Regression e2e: 74_run_no_loader_env_leak, 80_feature_defines, 81_capability_binding, 83_feature_defines_propagate — PASS.
• Self-host build at v0.0.73 — clean.

The Windows link/run half (clang-cl linking libopenblas.lib, DLL loading at launch, cblas_dgemm result) is verified in mcpp-index CI — Phase D of the design doc.

See .agents/docs/2026-06-29-windows-runtime-dll-deployment-and-openblas.md.