feat(quality): resolution/quality presets + Auto from pre-flight

Add a host-global quality knob (Discord-style) so the sharer can trade
resolution + bitrate for upload bandwidth. Quality is host-global by
design: one encode pipeline fans out to every viewer, so per-viewer
quality is out of scope (it would kill the broadcast fanout).

- New `--quality source|high|medium|low|auto` (ValueEnum) bundling a
  (max-height, bitrate, fps) tuple per preset; `auto` derives the preset
  from the saved bandwidth pre-flight (safe_mbps / viewer cap), falling
  back to `medium` when unmeasured. Default is auto; the interactive
  Host branch shows a picker when --quality is omitted (mirrors pick_app).
- `--max-height N` raw override; `--bitrate`/`--framerate` changed to
  Option so an explicit flag overrides just that field of the preset
  (precedence rule), leaving the rest of the preset intact.
- host/quality.rs: Preset table + resolve(); pure resolve_auto() split
  from the config read for testability. 5 unit tests lock preset
  pass-through, the Auto ladder, the unmeasured fallback, and override
  precedence.
- pipeline::build_args inserts `videoscale ! video/x-raw,height=N,
  pixel-aspect-ratio=1/1,width=[2,8192,2]` only for non-Source presets.
  PAR 1/1 forces a proportional downscale (without it videoscale keeps
  full width and squashes PAR — no bandwidth win); the even-stepped width
  range + even-rounded height satisfy H.264 4:2:0. EffectiveQuality is
  threaded capture -> wayland/x11 -> pipeline; max_viewers is now sized
  against the effective (post-preset) bitrate.
- Banner gains a quality line (preset label + ≤Np/kbps/fps + provenance).
- deps.rs checks `videoscale`; smoke-pipeline.sh adds a 1080->480
  downscale check asserting an even width below source.
- README: --quality preset table, Auto behavior, host-global note,
  --max-height/--bitrate/--framerate override precedence.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

README.md

@@ -31,6 +31,9 @@ Working:
 - First-run upstream bandwidth pre-flight, persisted to
   `~/.config/pixelpass/config.toml` and used to auto-size the default
   viewer cap
+- Quality presets (`--quality source|high|medium|low|auto`) that trade
+  resolution + bitrate for upload bandwidth, plus an `Auto` mode that
+  derives quality from the bandwidth pre-flight
 
 Not yet built (deferred, not blocking):
 - Per-monitor selection on a multi-monitor X11 host — `ximagesrc` grabs the
@@ -242,7 +245,37 @@ message and the host keeps running.
 
 For more viewers, drop the per-viewer bitrate: e.g. `pixelpass
 --bitrate 2500 --max-viewers 4` fits four 2.5 Mbps streams in roughly
-12 Mbps of upstream.
+12 Mbps of upstream. The `--quality` presets below are the friendlier
+way to do the same thing.
+
+## Quality
+
+`--quality <preset>` bundles a max video height, bitrate, and framerate —
+resolution is a quality-per-bitrate knob, so the three only make sense
+together. Quality is **host-global**: one encode pipeline fans out to every
+viewer, so the sharer picks one quality for everyone (per-viewer quality
+would need per-viewer encodes, which kills the fanout).
+
+| Preset   | Max height        | Bitrate   | fps |
+|----------|-------------------|-----------|-----|
+| `source` | native (no scale) | 6000 kbps | 30  |
+| `high`   | 1080p             | 4000 kbps | 30  |
+| `medium` | 720p              | 2500 kbps | 30  |
+| `low`    | 480p              | 1000 kbps | 30  |
+| `auto`   | derived (below)   | derived   | 30  |
+
+`auto` (the default) picks the highest preset whose bitrate fits your
+measured safe upstream divided by the viewer cap — so quality is sized for
+the worst case, since it's baked in when capture starts and can't drop when
+a second viewer joins. With no `--max-viewers`, it sizes for a single
+viewer. If there's no bandwidth measurement yet, `auto` falls back to
+`medium` (run `pixelpass --reconfigure` to measure). In the interactive
+menu, omitting `--quality` shows a picker instead of assuming `auto`.
+
+Downscaling preserves the source aspect ratio with square pixels and snaps
+to even dimensions (H.264 requires them). Power users can override
+individual fields: `--max-height N`, `--bitrate N`, and `--framerate N`
+each take precedence over the chosen preset's value for that field.
 
 ## Known limitations and gotchas