fix(housekeeping): unaligned read UB + recv-drop parity; dedup mmsghdr; doc fixes

From a bug-hunt + unsafe-audit pass (4 reviewers + adversarial verify). It
confirmed ZERO real bugs in the recent batched/paced data-plane work — these are
the surfaced cleanups + one genuine soundness fix:

- SOUNDNESS (reduce unsafe): inject/gamepad.rs::pump_ff did `ptr::read` of an
  InputEventRaw (align 8, holds a timeval) out of a 1-aligned [u8; N] buffer — UB
  per the reference (x86_64 tolerates it, but it can miscompile under LTO). Use
  ptr::read_unaligned + a SAFETY note. Zero behavior change.
- recv parity: recv_batch (recvmmsg) didn't drop an oversized/truncated datagram
  the way scalar recv does — poll_frame now skips a message whose len fills the
  buffer (> MAX_DATAGRAM_BYTES), matching recv's `n >= RECV_BUF` drop. (AEAD
  already rejected these on encrypted sessions; this restores the documented
  invariant on the batched path.)
- dedup unsafe FFI: factor the identical mmsghdr-from-iovec construction out of
  send_batch + recv_batch into one `mmsghdrs()` helper — the raw-pointer
  scaffolding + its lifetime SAFETY note now live in one place.
- docs: TARGET_SOCKBUF no longer calls paced sending future work (it landed,
  m3.rs::paced_submit); gamescope.rs input is no longer "(TODO)" (wired +
  live-validated); the PUNKTFUNK_PERF `wire_mbps` field is renamed `tx_mbps` and
  noted as attempted/sealed bytes (send_dropped shows what didn't reach the wire).

Full suite (35 + loopback round-trip + 6) + clippy + fmt green.

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

crates/punktfunk-core/src/session.rs

@@ -231,6 +231,12 @@ impl Session {
             let i = self.recv_idx;
             self.recv_idx += 1;
             let len = self.recv_lens[i];
+            // An oversized datagram fills the whole buffer (recvmmsg truncates + caps msg_len at the
+            // buffer size) — drop it rather than hand up a truncated, corrupt packet, mirroring the
+            // scalar `recv`'s `n >= RECV_BUF` check.
+            if len > MAX_DATAGRAM_BYTES {
+                continue;
+            }
             let pkt = match self.open_from_wire(&self.recv_scratch[i][..len]) {
                 Ok(p) => p,
                 Err(_) => continue,