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/host_test.cpp

@@ -1,5 +1,5 @@
-// Minimal stand-in for the not-yet-built server: dlopen()s a compiled
-// template .so and calls its exported render entry point directly, the
+// Minimal stand-in for a server: dlopen()s a compiled
+// template .jtc and calls its exported render entry point directly, the
 // way a persistent content-provider process would (no process spawn per
 // request, no re-parsing the .jet source - both the compile step AND
 // the shared-object load happen once, ahead of time).
@@ -14,7 +14,7 @@ typedef void (*RenderFn)(jet::Global &, coreutils::MString &);
 
 int main(int argc, char **argv, char **envp) {
    if (argc < 3) {
-      std::cerr << "usage: host_test <module.so> <symbol>" << std::endl;
+      std::cerr << "usage: host_test <module.jtc> <symbol>" << std::endl;
       return 1;
    }
    void *handle = dlopen(argv[1], RTLD_NOW);
@@ -31,7 +31,7 @@ int main(int argc, char **argv, char **envp) {
    }
 
    // Simulate the host calling the SAME module twice, for two different
-   // "requests", to prove the .so is reusable across calls without being
+   // "requests", to prove the .jtc is reusable across calls without being
    // reloaded - the actual point of a persistent content provider.
    for (int i = 0; i < 2; ++i) {
       jet::Global global(envp);