feat: HDR Step-0 colour-metadata transport + security-audit hardening

Two strands, entangled in punktfunk1.rs, committed together (one builds-green tree).

HDR pipeline Step 0 — glass-to-glass colour-metadata transport (docs/hdr-pipeline-plan.md):
- Protocol/ABI: ColorInfo on the Welcome + a 0xCE HdrMeta datagram carry the source colour
  space + HDR10 static mastering metadata (quic.rs, abi.rs connect_ex5 fixing caps=0).
- New platform-independent, unit-tested HDR static-metadata helpers (hdr.rs): chromaticities
  (1/50000), mastering luminance (0.0001 cd/m2), MaxCLL/MaxFALL in HDR10/ST.2086 units.
- Capture/encode hooks (capture.rs, encode.rs set_hdr_meta) + Linux client / probe plumbing.

Security-audit hardening — top 3 from docs/security-review.md, each adversarially verified:
- #1 [HIGH] Secret file permissions. The host key.pem/cert.pem and both trust stores are now
  written owner-only: 0600 + dir 0700 on Unix (mirrors mgmt_token), best-effort
  SYSTEM/Administrators/OWNER-only icacls DACL on Windows (%ProgramData% is Users-readable).
  Closes a local key-disclosure -> host-impersonation gap. New gamestream::{create_private_dir,
  write_secret_file} + a 0600 regression test.
- #2 [HIGH] Native SPAKE2 PIN is single-use. The PIN is consumed the moment the host sends its
  key-confirmation (which lets the client test its one guess), before reading the proof, so any
  completed attempt -- right OR wrong -- disarms the window. A wrong PIN isn't observable
  host-side (the client aborts before sending its proof), so consuming on first attempt is what
  delivers the documented "one online guess" instead of an unbounded brute-force of the static
  4-digit PIN. Test verifies single-use.
- #3 [MEDIUM] RTSP packetSize is bounded ([64,2048] in stream_config) and VideoPacketizer::new
  uses saturating .max(1), killing a PRE-AUTH div-by-zero/underflow panic of the video thread.
  Tests for {0,15,16,17} + out-of-range rejection.

fmt + clippy -D warnings clean; full workspace test suite green (93 host tests).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

clients/probe/src/main.rs

@@ -402,6 +402,9 @@ async fn session(args: Args) -> Result<()> {
         frames = welcome.frames,
         compositor = welcome.compositor.as_str(),
         gamepad = welcome.gamepad.as_str(),
+        bit_depth = welcome.bit_depth,
+        color = ?welcome.color,
+        hdr = welcome.color.is_hdr(),
         "session offer"
     );
 
@@ -826,12 +829,20 @@ async fn session(args: Args) -> Result<()> {
         let conn2 = conn.clone();
         tokio::spawn(async move {
             use std::sync::atomic::Ordering::Relaxed;
+            let mut hdr_logged = false;
             while let Ok(d) = conn2.read_datagram().await {
                 if let Some((_, _, opus)) = punktfunk_core::quic::decode_audio_datagram(&d) {
                     a.fetch_add(1, Relaxed);
                     ab.fetch_add(opus.len() as u64, Relaxed);
                 } else if punktfunk_core::quic::decode_rumble_datagram(&d).is_some() {
                     r.fetch_add(1, Relaxed);
+                } else if let Some(meta) = punktfunk_core::quic::decode_hdr_meta_datagram(&d) {
+                    // HDR static metadata (0xCE). Log the first receipt so a loopback test can
+                    // assert the host sent it for an HDR session.
+                    if !hdr_logged {
+                        hdr_logged = true;
+                        tracing::info!(?meta, "HDR static metadata (0xCE)");
+                    }
                 } else if let Some(hid) = punktfunk_core::quic::HidOutput::decode(&d) {
                     // The DualSense feedback plane (lightbar / player LEDs / adaptive triggers).
                     // Log the first few so a playtest can see triggers/LEDs arrive without spam.