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>

crates/punktfunk-host/src/gamestream/cert.rs

@@ -33,12 +33,16 @@ impl ServerIdentity {
             (Ok(c), Ok(k)) if !c.trim().is_empty() && !k.trim().is_empty() => (c, k),
             _ => {
                 let (c, k) = generate()?;
-                fs::create_dir_all(&dir).ok();
-                fs::write(&cert_path, &c)
-                    .with_context(|| format!("write {}", cert_path.display()))?;
-                fs::write(&key_path, &k)
+                // The private key is the trust root for EVERY surface (TLS server cert, pairing
+                // signing, the QUIC identity clients pin) — write it owner-only (0600 / SYSTEM-only
+                // DACL) so a local user can't read it and impersonate the host. The dir is 0700.
+                super::create_private_dir(&dir).ok();
+                super::write_secret_file(&key_path, k.as_bytes())
                     .with_context(|| format!("write {}", key_path.display()))?;
-                tracing::info!(path = %cert_path.display(), "generated punktfunk host certificate (RSA-2048)");
+                // The cert is public (handed to clients), but write it owner-only too for consistency.
+                super::write_secret_file(&cert_path, c.as_bytes())
+                    .with_context(|| format!("write {}", cert_path.display()))?;
+                tracing::info!(path = %cert_path.display(), "generated punktfunk host certificate (RSA-2048, key 0600)");
                 (c, k)
             }
         };