jetc: rename --shared build output convention from .so to .jtc

Purely a naming-convention change per user request, not a format
change - the output is still a real ELF shared object (-shared -fPIC),
still dlopen()-able exactly as before. .jtc ("jet compiled") marks it as
an APIServer-loadable content module rather than a generic library.

build-shared.sh: usage text and STEM derivation
(basename "$OUTPUT" .so -> .jtc) both updated - the STEM change is the
one functional piece here, everything else in this commit is comments/
usage strings. Doc comments in jetc.cpp/JetCodegen.h/host_test.cpp/
api_test.cpp updated to match.

Verified live: rebuilt jetc and all five demo*.jtc modules, reran
api_test/host_test against demo3.jtc (still resolves jetc_api_demo3/
jetc_render_demo3 and renders correctly) - the extension is purely
cosmetic to dlopen(), as expected.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

.gitignore

@@ -6,7 +6,7 @@ docs/JetCore.dvi
 docs/JetCore.log
 docs/JetCore.toc
 
-# jetc build byproducts (compiled JET-to-C++ output, binaries, .so/.a artifacts).
+# jetc build byproducts (compiled JET-to-C++ output, binaries, .jtc/.a artifacts).
 # Generated .cpp files are always named "*.compiled.cpp" (see build-compiled.sh/
 # build-shared.sh) - deliberately not a blanket "*.cpp", which would also
 # swallow real source files added to jetc/ later (bit us once already: an
@@ -19,7 +19,11 @@ jetc/host_test
 jetc/api_test
 jetc/demo.cpp
 jetc/demo2.cpp
+jetc/demo4
+jetc/demo5
+jetc/demo6
+jetc/demo7
 jetc/*.compiled.cpp
 jetc/*.pic.o
-jetc/*.so
+jetc/*.jtc
 jetc/libCoreUtils_pic.a