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/JetCodegen.h

@@ -10,7 +10,7 @@ namespace jetc {
 
    // One `#api METHOD /path` directive from the top of the source template
    // (see jetc.cpp) - a compile-time-known route this template wants to
-   // serve once loaded as a .so. Purely descriptive data handed to Codegen;
+   // serve once loaded as a .jtc. Purely descriptive data handed to Codegen;
    // jetc itself never interprets the method/path beyond storing them.
    struct ApiRoute {
       std::string method;
@@ -34,7 +34,7 @@ namespace jetc {
       //
       // sharedMode == true: emits no main() at all. Instead the render
       // logic is exported as `extern "C" void <entrySymbol>(jet::Global
-      // &global, coreutils::MString &out)`, meant to be built into a .so
+      // &global, coreutils::MString &out)`, meant to be built into a .jtc
       // (see build-shared.sh) and dlsym()'d by a long-running host server -
       // one call renders one request into the caller-owned `out`, using
       // whatever Global the host passes in (its lifecycle, cgi/session
@@ -45,7 +45,7 @@ namespace jetc {
       // letting coreutils::Exception propagate to the caller) across the
       // dlopen() boundary is safe; this is a same-ABI plugin boundary, not
       // a foreign C ABI one. entrySymbol should be unique per compiled
-      // template (see jetc.cpp) so multiple .so's can coexist even if a
+      // template (see jetc.cpp) so multiple .jtc's can coexist even if a
       // host ever dlopen()s them RTLD_GLOBAL instead of keeping separate
       // handles.
       //