`core/download_engine/rate_limit.py` introduces a per-source
policy declaration: download_concurrency + download_delay_seconds.
Plugins declare via `RATE_LIMIT_POLICY` class attribute or a
`rate_limit_policy()` method.
Engine applies the declared policy to engine.worker at
register_plugin time — set_concurrency + set_delay get pushed
in automatically. Plugins without a declaration get the
conservative default (1 / 0). The set_engine callback fires
AFTER policy registration so config-driven sources (YouTube
reads user-tunable youtube.download_delay) can override.
Plan doc updated to reflect Phase D skip (search code is 90%
source-specific, not 60% — lifting it would be lossy or
bloated).
Pure additive — no plugin migrated yet. 8 tests pin the
resolution priority + engine wire-up + override semantics.
Suite still green (327 download tests).
13 tests pin slskd HTTP API contract: endpoint format
(`transfers/downloads/<username>` POST), payload shape
(slskd web-interface array format), id extraction from dict /
list / fallback responses, and the username-lookup fallback in
cancel_download when no username hint is provided.
Phase A of the download engine refactor — pinning current
behavior of every source BEFORE moving any code so the engine
extraction can't drift the per-source contract. Includes the
plan doc at docs/download-engine-refactor-plan.md.
Pure additive — no client code changes.
