TASK-030: _handler suffix renames + explicit webserver ctor

Rename the three error-page setters on `create_webserver` from the
`_resource` to the `_handler` suffix, retype them to take
`std::function<http_response(const http_request&)>` (return by value,
matching the on_* family from TASK-025/026), and mark
`webserver(const create_webserver&)` `explicit` so a stray
`webserver ws = some_create_webserver;` no longer compiles.

  - `not_found_resource`           -> `not_found_handler`
  - `method_not_allowed_resource`  -> `method_not_allowed_handler`
  - `internal_error_resource`      -> `internal_error_handler`

Surface changes:
  - Drop the `render_ptr` typedef; introduce a public `error_handler`
    typedef in `create_webserver.hpp` to keep the field/setter parameter
    types in sync.
  - Adapters in `webserver_impl::{not_found,method_not_allowed,
    internal_error}_page` now wrap the by-value handler result via
    `std::make_shared<http_response>(...)`; `force_our` semantics on
    `internal_error_page` preserved.
  - Constructor doc-comment replaced ("Keeping this non explicit on
    purpose...") with a PRD-NAM-REQ-004 trace.

Compile-time sentinel:
  - New `test/unit/create_webserver_explicit_test.cpp` pins both
    guarantees via static_asserts:
      * `!is_convertible_v<create_webserver, webserver>` (negative),
      * `is_constructible_v<webserver, const create_webserver&>` (positive),
      * positive SFINAE detectors for the three `_handler`-suffixed
        setters with the by-value handler signature,
      * negative SFINAE detectors confirming the `_resource`-suffixed
        setters are gone.
  - Wired into `test/Makefile.am` with empty LDADD (header-only compile
    test, following the `webserver_pimpl` precedent).

Mechanical migration across the call-site graph:
  - 250 implicit copy-init sites converted to direct-init
    (`webserver ws = create_webserver(...);` -> `webserver ws{...};`)
    across 45 files in `test/`, `examples/`, and `README.md`.
  - All 11 setter-call sites renamed.
  - 6 error-handler function/lambda bodies retargeted from
    `shared_ptr<http_response>` to `http_response` by value
    (custom_error.cpp, ws_start_stop.cpp, basic.cpp, create_webserver_test.cpp).
  - README error-page documentation updated to reflect the new
    `std::function<http_response(const http_request&)>` signature.

Acceptance gates (all empty as required):
  - `grep -rE '(not_found|method_not_allowed|internal_error)_resource'
     src/httpserver/*.hpp`
  - `grep -rEn '\brender_ptr\b' src/ test/ examples/`
  - `grep -rEn '\bwebserver\s+\w+\s*=\s*(httpserver::|ht::)?create_webserver\b'
     test/ examples/ README.md`
  - `grep -n 'NOLINT(runtime/explicit)' src/httpserver/webserver.hpp`

All 41 tests pass (40 prior + 1 new sentinel). cpplint clean on every
touched file.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: etr

README.md

@@ -30,7 +30,7 @@ class hello_world_resource : public httpserver::http_resource {
 };
 
 int main() {
-    httpserver::webserver ws = httpserver::create_webserver(8080);
+    httpserver::webserver ws{httpserver::create_webserver(8080)};
 
     auto hwr = std::make_shared<hello_world_resource>();
     hwr->disallow_all();

examples/allowing_disallowing_methods.cpp

@@ -87,7 +87,7 @@ class args_resource : public httpserver::http_resource {
 };
 
 int main() {
-    httpserver::webserver ws = httpserver::create_webserver(8080);
+    httpserver::webserver ws{httpserver::create_webserver(8080)};
 
     auto ar = std::make_shared<args_resource>();
     ws.register_path("/args", ar);

examples/args_processing.cpp

@@ -36,7 +36,7 @@ class user_pass_resource : public httpserver::http_resource {
 };
 
 int main() {
-    httpserver::webserver ws = httpserver::create_webserver(8080);
+    httpserver::webserver ws{httpserver::create_webserver(8080)};
 
     auto hwr = std::make_shared<user_pass_resource>();
     ws.register_path("/hello", hwr);

examples/basic_authentication.cpp

@@ -43,10 +43,10 @@ class hello_world_resource : public httpserver::http_resource {
 int main(int argc, char** argv) {
     std::ignore = argc;
 
-    httpserver::webserver ws = httpserver::create_webserver(atoi(argv[1]))
+    httpserver::webserver ws{httpserver::create_webserver(atoi(argv[1]))
         .start_method(httpserver::http::http_utils::INTERNAL_SELECT)
         .tcp_nodelay()
-        .max_threads(atoi(argv[2]));
+        .max_threads(atoi(argv[2]))};
 
     std::shared_ptr<httpserver::http_response> hello = std::shared_ptr<httpserver::http_response>(new httpserver::http_response(httpserver::http_response::string(BODY)));
     hello->with_header("Server", "libhttpserver");

examples/benchmark_nodelay.cpp

@@ -43,9 +43,9 @@ class hello_world_resource : public httpserver::http_resource {
 int main(int argc, char** argv) {
     std::ignore = argc;
 
-    httpserver::webserver ws = httpserver::create_webserver(atoi(argv[1]))
+    httpserver::webserver ws{httpserver::create_webserver(atoi(argv[1]))
         .start_method(httpserver::http::http_utils::INTERNAL_SELECT)
-        .max_threads(atoi(argv[2]));
+        .max_threads(atoi(argv[2]))};
 
     std::shared_ptr<httpserver::http_response> hello = std::shared_ptr<httpserver::http_response>(new httpserver::http_response(httpserver::http_response::string(BODY)));
     hello->with_header("Server", "libhttpserver");

examples/benchmark_select.cpp

@@ -43,8 +43,8 @@ class hello_world_resource : public httpserver::http_resource {
 int main(int argc, char** argv) {
     std::ignore = argc;
 
-    httpserver::webserver ws = httpserver::create_webserver(atoi(argv[1]))
-        .start_method(httpserver::http::http_utils::THREAD_PER_CONNECTION);
+    httpserver::webserver ws{httpserver::create_webserver(atoi(argv[1]))
+        .start_method(httpserver::http::http_utils::THREAD_PER_CONNECTION)};
 
     std::shared_ptr<httpserver::http_response> hello = std::shared_ptr<httpserver::http_response>(new httpserver::http_response(httpserver::http_response::string(BODY)));
     hello->with_header("Server", "libhttpserver");

examples/benchmark_threads.cpp

@@ -71,7 +71,7 @@ class image_resource : public httpserver::http_resource {
 };
 
 int main() {
-    httpserver::webserver ws = httpserver::create_webserver(8080);
+    httpserver::webserver ws{httpserver::create_webserver(8080)};
 
     auto ir = std::make_shared<image_resource>();
     ws.register_path("/image", ir);

examples/binary_buffer_response.cpp

@@ -56,9 +56,9 @@ int main() {
     // Create webserver with centralized authentication
     // - auth_handler: called before every resource's render method
     // - auth_skip_paths: paths that bypass authentication
-    webserver ws = create_webserver(8080)
+    webserver ws{create_webserver(8080)
         .auth_handler(auth_handler)
-        .auth_skip_paths({"/health", "/public/*"});
+        .auth_skip_paths({"/health", "/public/*"})};
 
     auto hello = std::make_shared<hello_resource>();
     auto health = std::make_shared<health_resource>();

examples/centralized_authentication.cpp

@@ -148,11 +148,11 @@ int main() {
     std::cout << "  /secure - Requires valid client certificate\n\n";
 
     // Create webserver with SSL and client certificate trust store
-    httpserver::webserver ws = httpserver::create_webserver(8443)
+    httpserver::webserver ws{httpserver::create_webserver(8443)
         .use_ssl()
         .https_mem_key("server_key.pem")      // Server private key
         .https_mem_cert("server_cert.pem")    // Server certificate
-        .https_mem_trust("ca_cert.pem");      // CA certificate for verifying client certs
+        .https_mem_trust("ca_cert.pem")};      // CA certificate for verifying client certs
 
     auto secure = std::make_shared<secure_resource>();
     auto info = std::make_shared<info_resource>();