Add pixelpass --doctor environment diagnostic

Screen-share failures are usually environment gaps, not pixelpass bugs —
most often a GPU/driver with no working VA-API H.264 encoder, so the
default vah264enc pipeline produces no video and the viewer "can't
connect." doctor probes the whole chain and prints one actionable report
so a remote tester can read it over a call instead of us guessing from
logs, and it validates any X11/Wayland test environment we stand up.

Checks (each a ✓/!/✗ line with a distro-aware install hint):
- display server (Wayland/X11 + session env), and the X server vendor/
  version so an xlibre server is distinguishable from stock Xorg
- capture: gst tools + the backend's source element (pipewiresrc/ximagesrc)
- encode: hardware H.264 (vah264enc + DRM render node + a VA-API H.264
  *encode* entrypoint parsed from vainfo) and the software x264 fallback
- mux/audio tail + pactl
- viewer player (mpv/vlc)
- network: binds a real endpoint and checks relay reachability

Unlike deps::check_host_binaries (bails on first miss), doctor runs every
check and reports them together. Closes with a specific hosting verdict and
exits non-zero on any hard failure so scripts/CI can gate. Pure seams
(vainfo entrypoint parse, summary tally, hosting verdict) are unit-tested;
deps.rs gained pub(crate) which/gst_element_exists/install-hint/distro
helpers so doctor reuses the same package-name knowledge.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/common/deps.rs

@@ -56,12 +56,7 @@ fn require(bin: &str) -> Result<PathBuf> {
 }
 
 fn require_gst_element(name: &str) -> Result<()> {
-    let ok = Command::new("gst-inspect-1.0")
-        .args(["--exists", name])
-        .status()
-        .map(|s| s.success())
-        .unwrap_or(false);
-    if !ok {
+    if !gst_element_exists(name) {
         bail!(
             "GStreamer element `{name}` not available.\n{}",
             install_hint_for_gst_element(name)
@@ -70,7 +65,17 @@ fn require_gst_element(name: &str) -> Result<()> {
     Ok(())
 }
 
-fn which(bin: &str) -> Option<PathBuf> {
+/// Whether a GStreamer element is registered, via `gst-inspect-1.0 --exists`.
+/// Non-bailing counterpart to [`require_gst_element`] for the `doctor` report.
+pub(crate) fn gst_element_exists(name: &str) -> bool {
+    Command::new("gst-inspect-1.0")
+        .args(["--exists", name])
+        .status()
+        .map(|s| s.success())
+        .unwrap_or(false)
+}
+
+pub(crate) fn which(bin: &str) -> Option<PathBuf> {
     let path = std::env::var_os("PATH")?;
     for dir in std::env::split_paths(&path) {
         let candidate = dir.join(bin);
@@ -81,7 +86,7 @@ fn which(bin: &str) -> Option<PathBuf> {
     None
 }
 
-fn install_hint_for_bin(bin: &str) -> String {
+pub(crate) fn install_hint_for_bin(bin: &str) -> String {
     let distro = detect_distro();
     let pkg = match bin {
         "gst-launch-1.0" | "gst-inspect-1.0" => match distro.as_deref() {
@@ -113,7 +118,7 @@ fn install_hint_for_bin(bin: &str) -> String {
     install_command(&distro, pkg)
 }
 
-fn install_hint_for_gst_element(name: &str) -> String {
+pub(crate) fn install_hint_for_gst_element(name: &str) -> String {
     let distro = detect_distro();
     let pkg = match name {
         "pipewiresrc" => match distro.as_deref() {
@@ -210,7 +215,7 @@ fn install_command(distro: &Option<String>, pkg: &str) -> String {
     format!("Install hint: {cmd}")
 }
 
-fn detect_distro() -> Option<String> {
+pub(crate) fn detect_distro() -> Option<String> {
     let contents = std::fs::read_to_string("/etc/os-release").ok()?;
     for line in contents.lines() {
         if let Some(rest) = line.strip_prefix("ID=") {