round 16: log authd child.wait exit status + dnsd send_to failures

Round 16 audit follow-up. Three small fixes:

1. local/recipes/system/redbear-authd/source/src/main.rs:328 — the
   validation-mode spawn-reaper thread did 'let _ = child.wait();'
   silently dropping the session child's exit status. Replaced with
   'match' that eprintln-logs both successful exit status and wait
   failure (with errno). Operators can now see when the session
   child exits abnormally in validation mode.

2. local/recipes/system/redbear-dnsd/source/src/main.rs:111 — the
   loopback DNS responder did 'let _ = socket.send_to(&reply, &src);'
   silently. Replaced with 'if let Err(e) = socket.send_to(...)' that
   eprintln-logs the failure with the source peer address. Previously
   DNS clients could fail to receive a valid reply with no visible
   error.

3. local/docs/SUPERSEDED-DOC-LOG.md — appended Round 16 entries
   documenting the authd/dnsd fixes plus the Round-16 audit findings
   that did not require code changes (acpi-rs AML 41 panics classified
   as internal-invariant, pcid panics already have messages, all stale
   CONFIG refs already retired, USB-daemon let _ = is in host-only test
   paths, sessiond can_* for sleep is honest stub, Mesa CS ioctl
   numbers verified no drift).

3 files changed, +27/-2.

local/docs/SUPERSEDED-DOC-LOG.md

@@ -202,3 +202,23 @@ stubbing. The first two are addressed by rounds 9–14. Mesa DRM and
 Wayland null-guard work remains (EGL platform, PRIME/dma-buf, sync objects,
 single-context CS, libwayland null safety) and is tracked in
 `local/docs/3D-DESKTOP-COMPREHENSIVE-PLAN.md` § current state.
+
+## Round 16 — More `let _ =` fixes + sessiond sleep-probe design (2026-07-27)
+
+| Round | Item | Resolution commit | Note |
+|-------|------|---------------------|------|
+| 16 | `local/docs/SUPERSEDED-DOC-LOG.md` self-update | this entry | R15 entries appended (greeter/netctl/hotplugd let-underscore, acmd setrens security, README null+8 contradiction) |
+| 16 | `local/recipes/system/redbear-sessiond/manager.rs` `can_*` for hibernate/sleep | `7a5b5963c4` | Block comment documents future-work probe design: when `/scheme/sys/sleep` lands, mirror the `kstop_writable()` pattern used by `can_power_off/can_reboot/can_suspend`. No code change yet — kernel support doesn't exist. |
+| 16 | `local/recipes/system/redbear-authd/main.rs:328` child.wait | (this commit) | Replaced `let _ = child.wait()` with `match` that eprintln-logs exit status (validation-mode spawn reaper) |
+| 16 | `local/recipes/system/redbear-dnsd/main.rs:111` socket.send_to | (this commit) | Replaced `let _ = socket.send_to(...)` with `if let Err(e) = ...` logging the loopback failure with `src` peer address |
+
+### Round 16 — Audit findings with NO code changes needed
+
+| Category | Outcome |
+|----------|---------|
+| **acpi-rs AML 41 bare panics** | Audit classified all 41 sites as internal-invariant (firmware table validation, field/destructure safety). Adding diagnostic strings is a one-shot pure-text edit; tracked for a future round. |
+| **pcid IRQ/MSI-X panics** | All already carry diagnostic messages (`panic!("{driver}: failed to open IRQ file: {err}")`, etc.). The remaining concern is whether they should be `Result` (returns) — deferred; current panic behavior is correct for non-recoverable internal conditions. |
+| **Stale CONFIG references** | All `redbear-kde`/`redbear-minimal`/`redbear-wayland-guard`/`redbear-kde-session`/`pcid-spawner` are in retired/removed context; no live stale references remain. |
+| **USB daemon `let _ =` in ecmd/ftdi/usbaudiod** | All `#[cfg(not(target_os = "redox"))]` host-test paths only; never executed on Redox target. |
+| **`redbear-sessiond` can_hibernate/can_hybrid_sleep/can_suspend_then_hibernate/can_sleep hardcoded `"na"`** | Honest stub — correct D-Bus signal for missing kernel capability. Documented in module comment for future maintenance. |
+| **Mesa CS ioctl numbers** | Verified: `DRM_IOCTL_BASE + 31` / `+ 32` in `26-cs-submit-bidirectional-seqno.patch` match `redox-drm/scheme.rs` constants exactly. No drift. |