core: add the download engine — task machine, DownloadHandle, Engine

Stage 8 of the CORE build order: the bodies behind the DownloadSpec /
callback API reviewed in core/docs/engine-api-m1.md. Wires probe -> segment
workers -> WriteBuffer -> SparseFile -> .veloxpart.meta -> retry/backoff ->
SegmentBudget -> RateLimiter -> callbacks into one event-driven machine.

- Engine (src/engine.cpp): owns HttpClient, Prober, SegmentBudget,
  RateLimiter and one timer jthread (min-heap of scheduled fns). start()
  builds a task and returns a DownloadHandle; ~Impl quiesces every task
  before joining the timer so no callback fires during teardown.

- DownloadTaskState (src/task/download_task.cpp): one `mu` task lock; a
  shared_mutex over the worker map for the curl write path; callbacks
  collected under `mu` and fired after release via a separate deferred
  queue; weak_from_this() in every async hop. State machine over the
  CORE-owned EngineState subset, auto-pause on 401/407 and on a 200 where
  206 was expected, validated resume via If-Range.

- digest (src/task/digest.cpp): OpenSSL EVP hash_file() for the optional
  post-download checksum; links OpenSSL::Crypto PRIVATE.

- Segmenter::release_segment(): hand a paused segment back to the pool
  unassigned so resume's assign_slot() picks it up instead of splitting a
  still-"assigned" range and orphaning its front half.

- DownloadHandle now names the real control block (vdm::task::
  DownloadTaskState, defined only in the engine TU) via a namespace-scope
  fwd decl and a public-but-effectively-engine-only ctor, replacing the
  nested State/friend pair. Every public signature is unchanged; DAEMON
  (vdm-79) confirmed sched/ names only the public API.

Fixes found while building the end-to-end suite (tests/task/engine_test.cpp,
9 cases against tools/testserver, green under ASan/UBSan and TSan):
- a dropped connection lost its unflushed WriteBuffer tail while advance()
  had already counted those bytes as done -> a retry resumed past an
  unwritten hole. Flush on the failure path.
- when the byte counters hit total while other workers were still live,
  teardown dropped their buffered tails. Now: cancel them and let each
  worker's own seg_finished drain it (the `assembling` state), last one
  starts verification -- no cross-thread buffer access.
- seg_head() let a 401 with credentials present abort before libcurl's
  resend; now it proceeds once and acts on the final status.

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

core/tests/CMakeLists.txt

@@ -34,6 +34,13 @@ vdm_add_test(veloxcore_engine_api_test task/api_compiles_test.cpp)
 vdm_add_test(veloxcore_token_bucket_test rate/token_bucket_test.cpp)
 
 set(_testserver ${CMAKE_SOURCE_DIR}/tools/testserver/testserver.py)
+vdm_add_test(veloxcore_engine_test task/engine_test.cpp)
+target_include_directories(veloxcore_engine_test PRIVATE ${CMAKE_CURRENT_SOURCE_DIR}/net ${CMAKE_SOURCE_DIR}/core/src)
+if(EXISTS ${_testserver})
+    target_compile_definitions(veloxcore_engine_test PRIVATE VDM_TESTSERVER_PY="${_testserver}")
+    set_tests_properties(veloxcore_engine_test PROPERTIES TIMEOUT 300)
+endif()
+
 foreach(net_it http_client probe)
     vdm_add_test(veloxcore_${net_it}_test net/${net_it}_test.cpp)
     target_include_directories(veloxcore_${net_it}_test