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>

src/interactive.rs

@@ -3,7 +3,7 @@ use dialoguer::{Input, Select, theme::ColorfulTheme};
 use iroh_tickets::endpoint::EndpointTicket;
 use std::str::FromStr;
 
-use crate::cli::Cli;
+use crate::cli::{Cli, Quality};
 use crate::common::{bandwidth, config};
 use crate::{host, viewer};
 
@@ -27,6 +27,9 @@ pub async fn run(cli: Cli) -> Result<()> {
             if cli.app.is_none() {
                 cli.app = pick_app(&theme)?;
             }
+            if cli.quality.is_none() {
+                cli.quality = Some(pick_quality(&theme)?);
+            }
             host::run(cli.into_host_opts(true)).await
         }
         _ => {
@@ -80,6 +83,42 @@ fn pick_app(theme: &ColorfulTheme) -> Result<Option<String>> {
     }
 }
 
+/// Picker for the encode quality preset. Mirrors [`pick_app`]; bypassed when
+/// `--quality` was given on the CLI. Quality is host-global (the same stream
+/// fans out to every viewer), so this is the one choice that sets it for all.
+fn pick_quality(theme: &ColorfulTheme) -> Result<Quality> {
+    eprintln!();
+    eprintln!("Quality");
+    eprintln!("───────");
+    eprintln!("Lower presets trade resolution + bitrate for less upload usage.");
+    eprintln!("The same quality is sent to every viewer.");
+    eprintln!();
+
+    // Order mirrors the labels below; index maps back to a Quality.
+    let choices = [
+        Quality::Auto,
+        Quality::Source,
+        Quality::High,
+        Quality::Medium,
+        Quality::Low,
+    ];
+    let items = [
+        "Auto — pick from my measured upstream (recommended)",
+        "Source — native resolution, 6000 kbps",
+        "High — up to 1080p, 4000 kbps",
+        "Medium — up to 720p, 2500 kbps",
+        "Low — up to 480p, 1000 kbps",
+    ];
+
+    let choice = Select::with_theme(theme)
+        .with_prompt("What quality should the viewer(s) get?")
+        .items(&items)
+        .default(0)
+        .interact()?;
+
+    Ok(choices[choice])
+}
+
 /// `pixelpass --reconfigure` entry point: unconditionally re-run the
 /// bandwidth pre-flight test, save the result, and return. Used to
 /// refresh a stale measurement (e.g. user moved house, changed ISP).