core: net/http_client — libcurl multi wrapper (stage 2)

One HttpClient owns a small pool of workers, each with its own CURLM; an
easy handle lives on one worker for its life. Public start/pause/resume/
cancel enqueue a command + curl_multi_wakeup(); callbacks (on_head /
on_data / on_finished) run on the worker thread and return a DataAction
(proceed / pause / abort). Covers redirects (final-response head only),
ranges (inclusive ByteRange -> CURLOPT_RANGE), proxy/SOCKS5, basic/digest
auth, cookies, verbatim headers, stall detection, a coarse recv-rate cap,
and a curl_share DNS/TLS cache across workers. CURLcode + HTTP status ->
vdm::Error in net/curl_error. A probe is on_head returning abort: it
finishes successfully (head_complete), not canceled.

Tests drive tools/testserver: full GET, ranged 206, redirect chain, 404
-> not_found, connection refused -> connect_failed, HEAD probe + ranged
0-0 probe (no body), cancel mid-transfer, pause/resume completes. Skip
cleanly if testserver isn't in the tree.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01HPPSGhiArbvQgwC2DNiURS

core/CMakeLists.txt

@@ -5,16 +5,20 @@
 # `add_subdirectory(core)` in the root CMakeLists.txt (see core/docs/pkg-requests-m1.md).
 
 find_package(Threads REQUIRED)
+find_package(CURL 8.0 REQUIRED)
 
 add_library(veloxcore STATIC
     src/util/error.cpp
     src/util/log.cpp
     src/util/thread_pool.cpp
+    src/net/curl_error.cpp
+    src/net/http_client.cpp
 )
 add_library(velox::core ALIAS veloxcore)
 
-target_include_directories(veloxcore PUBLIC
-    ${CMAKE_CURRENT_SOURCE_DIR}/include
+target_include_directories(veloxcore
+    PUBLIC  ${CMAKE_CURRENT_SOURCE_DIR}/include
+    PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/src   # net/*.cpp -> "net/curl_error.hpp"
 )
 
 target_compile_features(veloxcore PUBLIC cxx_std_23)
@@ -25,9 +29,9 @@ target_compile_options(veloxcore PRIVATE
     -Wall -Wextra -Wpedantic -Werror
 )
 
-target_link_libraries(veloxcore PUBLIC Threads::Threads)
+target_link_libraries(veloxcore PUBLIC Threads::Threads CURL::libcurl)
 
-# Later stages add: find_package(CURL 8.0) for net/, find_package(OpenSSL) for meta/.
+# Later stages add: find_package(OpenSSL) for meta/ (streaming SHA-256 + resume CRC).
 
 if(VELOX_BUILD_TESTS)
     add_subdirectory(tests)