core: post the engine API for DAEMON review (pre-stage-7)

The download entry point the AGENT-CORE brief asked for on day one and
that slipped. DAEMON has an RPC surface and a store and, until this is
agreed, nothing in velox::core to call.

vdm/task/download.hpp — DownloadSpec (the resolved subset DAEMON hands in:
absolute save_path, verbatim browser headers, requested segments/buffer,
optional probe_hint / checksum / auth, allow_resume), EngineState (the
CORE-owned subset of the wire TaskState), Progress / SegmentProgress,
DownloadCallbacks (on_progress <=4 Hz, on_state for every transition
incl. auto-pauses, on_auth_required, on_decision_needed, on_finished
last), DownloadHandle (pause/resume/cancel — idempotent per the ADR 0013
signature — plus provide_auth / decide / refresh_url, and synchronous
state()/progress() snapshots).

vdm/engine.hpp — Engine: start(spec, callbacks) -> handle, segment_budget()
(DAEMON's sched/ admission surface, ADR 0011), live connection.* setters,
a standalone probe() on the pool outside the segment budget.

core/docs/engine-api-m1.md — the review doc: field semantics, the state
machine, threading/lifetime rules (which thread callbacks arrive on, what
is legal from inside one, handle/engine lifetime), the shared-`paused`
idempotency contract as a signature, and five open questions for DAEMON.

Value types compile and are covered by api_compiles_test; Engine /
DownloadHandle bodies land in stage 8, built against whatever DAEMON
signs off here.

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

core/include/vdm/README.md

@@ -1,8 +1,12 @@
 # `libveloxcore` — public API
 
-**Status: M1 in progress.** Only `util/` is landed. The download-facing API
-(`DownloadSpec`, `DownloadTask`, probe, typed callbacks) arrives with later stages and
-is reviewed by DAEMON before M2 (AGENT-CORE DoD).
+**Status: M1 in progress.** `util/`, `net/` (http_client, probe, url, content_disposition),
+`io/` (sparse_file, write_buffer), `meta/veloxpart`, and `segment/` (segmenter, budget)
+are landed. The **download entry point** — `vdm::Engine`, `vdm::task::DownloadSpec` /
+`DownloadHandle` / `DownloadCallbacks` — is sketched in `vdm/engine.hpp` and
+`vdm/task/download.hpp` and **out for DAEMON review**: see
+[`core/docs/engine-api-m1.md`](../../docs/engine-api-m1.md). Bodies land in CORE stage 8;
+build against the value types now.
 
 Layering (CLAUDE.md §3): this library knows nothing about JSON, SQL, Qt, or RPC. Input is
 a spec value; output is bytes on disk plus typed callbacks. DAEMON projects engine state