Phase 6.3: TCP window management — track guest window, advertise host kernel rcv-space

[dpsoft]

Status: DRAFT. Stacked on PR #74 (Phase 6.2 async connect).

What this branch does

Stops ignoring the guest's advertised window and stops hardcoding our own.

• `TcpNatEntry` tracks per-flow `guest_window: u32` + `guest_window_scale: u8`.
• `handle_tcp_frame` parses the WindowScale option from guest SYN, stashes it; updates `guest_window` on every incoming frame.
• `build_tcp_packet_static` signature now takes `(window_len, window_scale)` — caller decides per frame.
• SYN-ACK negotiates `window_scale: Some(7)` (passt's default; 128× scaling, extends window from 64 KiB to 8 MiB).
• `relay_tcp_nat_data` gates host→guest sends on `entry.guest_window` so the guest sees real backpressure.
• Outgoing frames advertise `host_recv_window(fd)` — derived from `getsockopt(TCP_INFO).tcpi_rcv_space` — instead of a hardcoded 65535.

Severity

Medium — perf gap. Today the relay emits `window_len: 65535, window_scale: None` on every outgoing frame, ignores the guest's advertised window entirely, and never honors guest backpressure. Throughput is capped at 64 KB / RTT regardless of bandwidth, and `inject_to_guest` could grow unbounded if the guest is slow.

Bench evidence (`bench-compare.sh --baseline 47868f0 --skip-vm`)

Bench	Baseline	HEAD	Note
`tcp_bulk_throughput_constrained_window/4096`	—	2.65 ms	new
`tcp_bulk_throughput_constrained_window/16384`	—	1.89 ms	new
`tcp_bulk_throughput_constrained_window/65536`	—	1.77 ms	new
`tcp_inbound_syn_ack_transition`	68 µs	51.6 µs	-24%
`process_syn`	33 µs	28 µs	-15%
`tcp_bulk_throughput_1mb`	62 ms	61.7 ms	parity
`poll_with_n_mixed_flows/999`	299 µs	10.1 µs	-97 % held
`port_forward_accept_latency`	50.1 ms	206 µs	held from #72

Wall-clock vs master

Metric	Master	This branch	Δ
TCP g2h throughput	1885 Mbps	5320 Mbps	+182 % (2.8×)
TCP bulk-g2h	1565 Mbps	4775 Mbps	+205 % (3.0×)
TCP CRR p50	~10 ms	~10.1 ms	parity
TCP RR p50	2 µs	2 µs	parity

Commits (10 + plan)

1. `b8772ca` — docs: Phase 6.3 TDD plan
2. `655f01f` — feat: TcpNatEntry tracks guest_window + guest_window_scale
3. `f4ff6a2` — feat: parse guest's window_scale on SYN, store on flow
4. `2260c61` — feat: track guest's advertised window on every incoming frame
5. `6541ba7` — refactor: build_tcp_packet_static takes (window_len, window_scale)
6. `b46085c` — feat: advertise host-kernel-derived window on outgoing frames
7. `b8d6716` — test: pin tcp_advertised_window_tracks_guest_buffer (BROKEN_ON_PURPOSE → flips at Supress macOS warnings #7)
8. `e74cf83` — feat: gate host→guest send on guest's advertised window
9. `a0cd92f` — test: pin tcp_window_scale_negotiated_in_synack
10. `aad52d9` — bench: tcp_bulk_throughput_constrained_window parametric

Validation

Suite	Status
`cargo fmt --all -- --check`	✅
`cargo clippy --workspace --all-targets --all-features -- -D warnings`	✅
`cargo test --test network_baseline`	✅ 24/24
`cargo test --test network_baseline --features bench-helpers -- --test-threads=1`	✅ 26/26
`cargo test --lib network`	✅ 25/25
`cargo bench --bench network --features bench-helpers --no-run`	✅
`cargo build --release`	✅