feat(downloader): resume failed downloads from completed segments across runs

Fingerprint-gated reuse of atomically-published segment files (DASH/ISM/HLS) behind a continue_downloads config option and one-off dl --continue-downloads flag.

docs/guide/cli-reference.md

@@ -202,6 +202,7 @@ order the service used, then renumbers the episodes into the order you asked for
 | `--workers` | downloader default | Per-track download threads. |
 | `--adaptive-workers` | off | Scale per-track segment workers dynamically (up to the `--workers` cap) based on measured CDN throughput and errors. |
 | `--download-processes` | `1` | Split large segment batches (24+) across this many download processes to exceed the single-process throughput cap on multi-gigabit connections. Ignored while a speed limit is set: the cap is one shared budget, which extra processes cannot share, so the download stays in a single process. |
+| `--continue-downloads` | off | Keep completed segment files across runs and resume a previously failed download. One-off enable of the [`continue_downloads`](../reference/configuration/download.md#continue_downloads) config option. |
 | `--downloads` | `1` | Number of tracks downloaded concurrently. |
 | `--speed-limit` | unlimited | Cap total download speed across all threads and tracks, e.g. `500k`, `5M`, `1.5G` or plain bytes/sec. Values are bytes, not bits (`5M` = 5.0 MB/s). `off` disables a configured limit. |
 | `--no-cache` | off | Bypass the title cache. |

docs/guide/downloading.md

@@ -613,6 +613,7 @@ Two related flags:
 | `--workers N` | Threads used per track for segment downloads. Default depends on the downloader. |
 | `--adaptive-workers` | Opt-in: start with a moderate per-track worker count and ramp it up or back off based on measured CDN throughput and errors, capped at `--workers`. Off by default (fixed worker count). |
 | `--download-processes N` | Opt-in: split a large segment batch across `N` download processes, each with its own worker pool. A single process tops out around 1.3 Gb/s (Python interpreter limit); `2` reaches 2.5GbE line rate on fast CDNs. Only engages for batches of 24+ segments. Ignored while `--speed-limit` (or serve's `global_speed_limit`) is set: the cap is one shared budget, which extra processes cannot share, so the download stays in a single process. Default `1`. |
+| `--continue-downloads` | Opt-in: keep completed segment files when a download fails so the next run resumes instead of restarting from zero. One-off enable of the [`continue_downloads`](../reference/configuration/download.md#continue_downloads) config option, which documents what can and cannot resume. |
 | `--downloads N` | Number of tracks downloaded concurrently. Default `1`. |
 | `--slow [MIN-MAX]` | Add a delay between titles to look more like a real device. `--slow` alone means 60-120s; `--slow 20-40` sets a custom range. Minimum 20s. |
 | `--no-cache` | Bypass the title cache for this download. |

docs/reference/configuration/download.md

@@ -92,6 +92,34 @@ dl:
     to set server-wide download defaults for every request it handles. Keys the API does not know
     are ignored there (`speed_limit`, for example; `serve` has its own `global_speed_limit`).
 
+## `continue_downloads`
+
+- **Type:** `bool`  ·  **Default:** `false`
+
+Keep a failed download's completed segment files so the next attempt resumes from where it
+stopped instead of restarting from byte zero.
+
+Only whole, fully downloaded segments are reused; partially written segments are always
+discarded, and no partial-byte (`Range`) resume across runs takes place. Reuse also
+requires proof that the remote content is unchanged: the manifest URL, segment count, and
+byte ranges must all match the previous run, otherwise the download silently restarts
+clean.
+
+!!! note "What can and cannot resume"
+    - Segmented DASH, ISM, and HLS tracks resume.
+    - Single-file (non-segmented) downloads and DASH tracks collapsed to a single ranged
+      download never produce segment files, so this option does not apply to them (the
+      downloader has its own in-flight partial-file handling for those).
+    - HLS resumes Widevine/PlayReady and unencrypted content only; AES-128 content always
+      restarts. An HLS failure during the post-download merge step also restarts.
+
+For a one-off resume without changing config, pass `dl --continue-downloads` on the
+command line (the flag can only enable resume, never disable it).
+
+```yaml title="Resume failed downloads"
+continue_downloads: true
+```
+
 ## `subtitle`
 
 - **Type:** `dict`  ·  **Default:** `{}`

docs/reference/configuration/index.md

@@ -31,7 +31,7 @@ Where the file lives, how unshackle discovers it, and how to edit it:
 |------|------|
 | [Directories](directories.md) | `directories`, `filenames` |
 | [Services & authentication](services.md) | `services`, `credentials`, `firefox_cookies`, `remote_services`, `serve` |
-| [Download & processing](download.md) | `dl`, `subtitle`, `audio`, `muxing`, `language_tags`, `dual_multi_mode` |
+| [Download & processing](download.md) | `dl`, `continue_downloads`, `subtitle`, `audio`, `muxing`, `language_tags`, `dual_multi_mode` |
 | [Output & naming](output.md) | `output_template`, `tag`, `tag_rules`, `tag_group_name`, `tag_imdb_tmdb`, `chapter_fallback_name`, `unicode_filenames` |
 | [DRM & CDM](drm.md) | `cdm`, `remote_cdm`, `decryption` |
 | [Network & proxy](network.md) | `network`, `headers`, `proxy_providers` |