docs(refactor): scope TCP buffer auto-tuning (Tier 3)

Add docs/refactor/tcp_buffer_autotuning.md — the implementation plan for
the separately-split Tier-3 buffer-accounting item (Linux DRS +
send-buffer auto-tuning), grounded in the shipped Tier-1 primitives.

Two tracks, symmetric to Tier-1's A/B:

- Track R (receive DRS, tcp_rcv_space_adjust): R1 receiver-side RTT
  estimator (the prerequisite — sender SRTT is unpopulated on a pure
  receiver; reuse the RFC 7323 TSecr echo), R2 RcvSpaceState + cumulative
  copied-bytes counter, R3 once-per-RTT trigger in receive(), R4 the BDP
  grow formula clamped at tcp.rmem.max. Actuator is the existing grow-only
  grow_rcv_wnd_max().

- Track S (send auto-tuning, tcp_sndbuf_expand): S1 a grow-only _sndbuf_auto
  folded into _effective_sndbuf(), S2 the 2*cwnd*mss expand policy fired
  from the ACK path. Actuator is the existing _charge_tx_buffer gate.

Records the load-bearing decision (auto-tuning assumes a small initial
buffer that grows, but Tier-1 chose large static defaults — Track S must
land paired with adopting a small tcp.wmem.default or it is cosmetic),
the sysctl set (tcp.rmem/tcp.wmem triples + tcp.moderate_rcvbuf, subsuming
that slice of Tier-2), the SOCK_*BUF_LOCK ⇔ option-unset lock reusing the
Tier-1 explicit-vs-None distinction, the no-GIL single-writer analysis for
every added field, the test strategy, and the recommended ordering. No
code.

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

Author: ccie18643

docs/refactor/host_refinements_backlog.md

@@ -358,6 +358,14 @@ self._signal_readable()
   the `SO_*BUF` options to work. **Global memory-pressure accounting
   (`tcp_mem`) is a documented non-goal** (kernel-memory management a
   userspace stack does not own).
+- **Full implementation plan scoped:** `docs/refactor/tcp_buffer_autotuning.md`
+  — Track R (receive DRS: R1 receiver-RTT estimator → R2 `RcvSpaceState` →
+  R3 trigger → R4 grow policy) and Track S (send auto-tuning: S1 auto bound →
+  S2 expand policy), both actuators reusing the shipped Tier-1 primitives
+  (`grow_rcv_wnd_max`, the `_effective_sndbuf()` gate). Registers the Tier-2
+  `tcp.rmem`/`tcp.wmem` triples + `tcp.moderate_rcvbuf`; the load-bearing
+  decision is pairing Track S with a small initial send default (§2/§7). No
+  code yet.
 
 ### R4 — IPv6 per-socket source filters = MLDv2 SSM track — DONE (P1-P5 shipped)