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>

jetc/api_test.cpp

@@ -1,7 +1,7 @@
 // Minimal dlsym() check for the jetc_api_<name> route-metadata export added
 // alongside jetc_render_<name> in --shared mode (see JetCodegen.cpp). Proves
 // a host can discover a compiled module's declared routes purely from the
-// .so itself, with no separate manifest file - the actual point of putting
+// .jtc itself, with no separate manifest file - the actual point of putting
 // routing metadata in an exported symbol instead of a sidecar file.
 #include <dlfcn.h>
 #include <iostream>
@@ -12,7 +12,7 @@ typedef const JetApiRoute *(*ApiFn)(int *count);
 
 int main(int argc, char **argv) {
    if (argc < 3) {
-      std::cerr << "usage: api_test <module.so> <api-symbol>" << std::endl;
+      std::cerr << "usage: api_test <module.jtc> <api-symbol>" << std::endl;
       return 1;
    }
    void *handle = dlopen(argv[1], RTLD_NOW);