lib: Further align FRCP with TCP RFCs

Only the HoL slot retransmits on RTO; non-HoL slots defer and rely on
SACK/RACK fast-rxm for recovery. Matches RFC 6298 §5.4 + RFC 8985 §3
and Linux tcp_retransmit_skb(head, 1). Eliminates the spurious-RTO
storm where ~50-66% of retransmits arrived as duplicates at the peer.

Co-fixes for three latent state-machine bugs that the previous
spurious-retx mask was hiding:

  - recovery_enter: seal recovery_high at the false→true edge only
    (RFC 6582 §3.2). Previously extended on every gap-SACK, which
    trapped the sender in NewReno indefinitely once any cum-ACK fell
    behind the moving recovery_high.

  - rtt_sample_eligible: drop the in_recovery super-gate. Karn at the
    per-slot SND_RTX|SND_TLP level is already correct (matches Linux).

  - rxm_due defer interval: use base RTO, not rto<<rto_mul. Inheriting
    HoL's backoff parked deferred slots tens of seconds in the future.

Bring RACK reorder-window scaling into RFC 8985 §7.2 compliance:
reo_wnd_mult widens at most once per RTT, gated via a srtt-elapsed
check in reo_wnd_on_dsack.

Stat refactor for clearer attribution: rename rxm_snd → rxm_rto (now
RTO-driven sends only), add rxm_nack and rxm_due_defer, split rxm_rcv
into total FRCT_RXM arrivals and rxm_dup_rcv (duplicates). Expose
rx/tx ring queue depths as RIB stats.

Signed-off-by: Dimitri Staessens <dimitri@ouroboros.rocks>
Signed-off-by: Sander Vrijders <sander@ouroboros.rocks>

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