TASK-015: http_request_impl skeleton (PIMPL split, structural only)

Move every backend-coupled member of http_request behind a
unique_ptr<detail::http_request_impl>. The public header
src/httpserver/http_request.hpp no longer includes <microhttpd.h>
or <gnutls/gnutls.h>; the only backend-typed names that remain
visible at the public surface are forward-declared (MHD_Connection*
in the HTTPSERVER_COMPILATION-gated private ctor, and a narrow
typedef forward-decl of gnutls_session_t for the still-public
get_tls_session() return type that TASK-019 will remove).

Outer keeps: path, method, content, version, content_size_limit,
plus the impl_ unique_ptr. Everything else (the live MHD_Connection*,
unescaper, file table, parsed-args / cookies / cert caches, the
MHD trampolines build_request_*, fetch_user_pass(),
populate_all_cert_fields()) lives on http_request_impl. Public
methods are one-line forwarders.

The dtor is out-of-line in http_request.cpp so unique_ptr<impl>
sees the complete impl type. Move ctor/assign remain defaulted
and operate on the unique_ptr.

A new sentinel test/unit/http_request_pimpl_test.cpp asserts that
http_request appears non-copy/non-move-constructible from external
scope (private moves), and locks sizeof(http_request) at <= 24
pointer widths. Currently 14 pointers (112 B on LP64).

Acceptance criteria all green:
- grep -E '#include <(microhttpd|gnutls/gnutls)\.h>' on the public
  header returns nothing.
- All v1 request-side tests pass (basic, file_upload,
  authentication, http_resource, threaded, nodelay, ws_start_stop,
  uri_log, etc. -- 27 PASS + 1 XFAIL header_hygiene umbrella,
  unchanged from baseline; the umbrella sweep is TASK-020).
- sizeof(http_request) = 14 * sizeof(void*); asserted in the new
  sentinel.
- noinst_HEADERS lists the impl header so it ships in the source
  tarball but never installs to $prefix/include.
- No test reaches across the boundary into http_request_impl.

Author: etr

specs/tasks/M3-request/TASK-015.md

@@ -8,11 +8,11 @@
 Move `http_request`'s backend-coupled members (`MHD_Connection*`, raw GnuTLS handle, computed caches) into `detail/http_request_impl.hpp` behind a `std::unique_ptr<http_request_impl>`. No API rename yet.
 
 **Action Items:**
-- [ ] Create `src/httpserver/detail/http_request_impl.hpp` (gated `HTTPSERVER_COMPILATION` only).
-- [ ] Move all backend-coupled state into the impl struct: `MHD_Connection* conn_`, `gnutls_session_t tls_session_`, parsed-args cache, headers cache, etc.
-- [ ] Public `http_request.hpp` declares `std::unique_ptr<http_request_impl> impl_;` and forward-declares the impl class.
-- [ ] Implement existing public methods as forwarders to `impl_->method()`.
-- [ ] Move `<microhttpd.h>`, `<gnutls/gnutls.h>` includes from public `http_request.hpp` into `http_request_impl.hpp` and `http_request.cpp`.
+- [x] Create `src/httpserver/detail/http_request_impl.hpp` (gated `HTTPSERVER_COMPILATION` only).
+- [x] Move all backend-coupled state into the impl struct: `MHD_Connection* conn_`, `gnutls_session_t tls_session_`, parsed-args cache, headers cache, etc.
+- [x] Public `http_request.hpp` declares `std::unique_ptr<http_request_impl> impl_;` and forward-declares the impl class.
+- [x] Implement existing public methods as forwarders to `impl_->method()`.
+- [x] Move `<microhttpd.h>`, `<gnutls/gnutls.h>` includes from public `http_request.hpp` into `http_request_impl.hpp` and `http_request.cpp`.
 
 **Dependencies:**
 - Blocked by: TASK-002, TASK-014
@@ -28,4 +28,4 @@ Move `http_request`'s backend-coupled members (`MHD_Connection*`, raw GnuTLS han
 **Related Requirements:** PRD-HDR-REQ-001..004
 **Related Decisions:** DR-003b, §4.2
 
-**Status:** Not Started
+**Status:** In Progress

specs/tasks/_index.md

@@ -97,7 +97,7 @@ Nominally: **13 sequential tasks**, each S–XL. Most other tasks parallelize of
 | TASK-012 | `http_response` fluent `with_*` setters | M2 | Done | TASK-009 |
 | TASK-013 | Remove `*_response` subclasses and dispatch virtuals | M2 | Done | TASK-009, TASK-010, TASK-011, TASK-012 |
 | TASK-014 | `webserver_impl` skeleton (PIMPL prep) | M3 | In Progress | TASK-002 |
-| TASK-015 | `http_request_impl` skeleton (PIMPL split) | M3 | Not Started | TASK-002, TASK-014 |
+| TASK-015 | `http_request_impl` skeleton (PIMPL split) | M3 | In Progress | TASK-002, TASK-014 |
 | TASK-016 | Per-connection arena for `http_request_impl` | M3 | Not Started | TASK-014, TASK-015 |
 | TASK-017 | `http_request` container getters return `const&` | M3 | Not Started | TASK-015 |
 | TASK-018 | `http_request` single-key getters return `string_view`, all const | M3 | Not Started | TASK-015, TASK-016 |

src/Makefile.am

@@ -23,7 +23,7 @@ libhttpserver_la_SOURCES = string_utilities.cpp webserver.cpp http_utils.cpp fil
 # noinst_HEADERS: shipped in the tarball but NEVER installed under $prefix/include.
 # Detail headers (httpserver/detail/*.hpp) live here so they cannot leak to
 # downstream consumers — the public surface comes in through <httpserver.hpp>.
-noinst_HEADERS = httpserver/string_utilities.hpp httpserver/detail/modded_request.hpp httpserver/detail/http_endpoint.hpp httpserver/detail/body.hpp httpserver/detail/webserver_impl.hpp gettext.h
+noinst_HEADERS = httpserver/string_utilities.hpp httpserver/detail/modded_request.hpp httpserver/detail/http_endpoint.hpp httpserver/detail/body.hpp httpserver/detail/webserver_impl.hpp httpserver/detail/http_request_impl.hpp gettext.h
 nobase_include_HEADERS = httpserver.hpp httpserver/body_kind.hpp httpserver/constants.hpp httpserver/create_webserver.hpp httpserver/webserver.hpp httpserver/http_utils.hpp httpserver/file_info.hpp httpserver/http_request.hpp httpserver/http_response.hpp httpserver/http_resource.hpp httpserver/feature_unavailable.hpp httpserver/iovec_entry.hpp httpserver/http_arg_value.hpp httpserver/http_method.hpp
 
 if HAVE_WEBSOCKET