fix: build the SONAME alias under explicit ninja goals (mcpp test) — 0.0.107

[Sunrisepeak]

A shared library that declares soname is written as bin/libX11.so but records
SONAME libX11.so.6. Both the linker resolving a transitive NEEDED and the loader
starting a binary look for the alias, never for the plain name — so the alias is
a prerequisite of anything that links the library, not a by-product of building
it.

It was modelled as a standalone ninja edge that nothing depends on, reachable
only through default. The 0.0.104 test batch (#274) made mcpp test pass
explicit goal targets so it could isolate per-test compiles, and that silently
stopped producing the alias. mcpp build and mcpp run drive default and
were unaffected, which is why this survived three releases: only a project
driven by mcpp test hits it.

The symptom lands three levels from the cause — a consumer failing to link with
libX11.so: undefined reference to xcb_connect (alongside libxcb.so.1 ... not found), or a test binary exiting 127. mcpp-index CI has been failing this way
on gui-stack, imgui-module and imgui-window since 0.0.104; two probe PRs pinning
only MCPP_VERSION over unmodified descriptors reproduced it at both 0.0.104 and
0.0.106, which is what ruled out the package-identity migration as the cause.

Hang the alias off the consumer's implicit inputs, beside the .so it already
lists. Correct under default and under explicit goals alike, and it pulls in
nothing that was not already being built.

e2e 64 covered only build + run — precisely the two paths that still worked.
It now also runs mcpp test from a clean target dir and asserts the alias
exists; without the fix that test binary fails with exit 127.

Verified: unit 37/37; e2e 64 red before / green after; the three real mcpp-index
members pass with mcpp test alone (gui-stack 5/5, imgui-module 1/1,
imgui-window 1/1). Remaining local e2e failures (22, 98, 141) reproduce
identically on the released 0.0.106 binary under the same MCPP_HOME — they are
this machine's gcc 15.1.0, not this change.