gettext: replace placeholder catalog tools with GNU implementations - #84

Why

Kandelo currently uses compact fallback applets for message catalogs. This PR adds GNU Gettext 1.0 as a normal tap formula so Kandelo users can run the real gettext, ngettext, msgfmt, and xgettext programs through Homebrew.

The formula intentionally installs only those four programs. It does not claim the rest of GNU Gettext's executable and library suite.

What changed

• builds the official GNU source with Kandelo's SDK and musl sysroot;
• consumes the tap's libiconv, libxml2, ncurses, and zlib formulas as explicit target dependencies;
• retains libxml2's complete static link closure in the required order: -lz -liconv -lm -lcharset -ldl -pthread;
• verifies the generated Makefile still contains the selected libxml2 archive, GNU libiconv include/library roots, and ordered static closure;
• builds the bundled libintl, libunistring, and libtextstyle components needed by the four selected programs;
• corrects gnulib's musl locale detection without identifying Kandelo as Linux;
• aligns libtextstyle's private tputs declaration with the ncurses ABI;
• validates all four programs as fork-free Kandelo Wasm artifacts with no retired Asyncify surface or leaked build paths;
• tests catalog extraction, compilation, character-set conversion, singular/plural lookup, ITS XML extraction, and styled output.

Review fix in this head

The libxml2 formula on tap main now uses GNU libiconv. Static Gettext links therefore need both libiconv and libcharset after libxml2. This head declares libiconv directly, passes its prefix to configure, supplies the complete ordered closure, and fails the build if configure drops or reorders those inputs.

Evidence

• Base: da5f694d1c9c01656bfd1beeb78a710af3a25d6e
• Head: 170ed74962d5fdba8a15c855ef37d5e83fac57f5
• Formula SHA-256: 20d9e7336a6865d60bf657b358982296be708dff55c7373ea5f0c226cb0003e1
• Ruby syntax, Homebrew style, and git diff --check pass.
• The generated-Makefile guards pass against a real Kandelo dev-shell configure result.
• Mutation checks reject a missing or wrong libiconv include root, a missing or wrong libiconv library root, a reordered closure, and a missing -lcharset.
• Independent exact-head rereview accepted this commit.

The earlier content-equivalent formula passed a full source build and literal brew test --verbose, including all four commands and the runtime cases listed above. That earlier build used ABI 18 and is historical source/runtime evidence only. A full build, bottle publication, and Node/Chromium pours must be repeated against the current ABI before this formula can ship.

Merge and publication gates

Do not merge this formula yet. It still depends on the main-repository SDK/configure fixes and the trusted Homebrew bottle publisher landing first. Publication must then build dependency-first from current-ABI bottles for libiconv, libxml2, ncurses, and zlib, followed by stock Homebrew pours in Node and Chromium.

This tap-only PR changes no Kandelo ABI, main-repository registry package, bottle block, archive, receipt, or generated Wasm artifact.