Configure TCP receive-window grow threshold per interface (#89)

[tinic]

Adds TCPGROWRTT=<ms> (1..65535) to interface files (#89). It sets the handshake round trip at or above which a socket on that interface grows its receive window from BSD_TCP_WINDOW_LAN (100352) to its maximum. Shaped after #109 (TCPACKMAX).

• Default stays 10 ms on every interface, WiFiPi included. There is no sana2_policy default; TCPGROWRTT=2 is an explicit opt-in.
• A NULL interface (or TCPGROWRTT unset) uses BSD_TCP_WINDOW_GROW_RTT_MS (10), never a configured value.
• Parsed in config_parse.c; 0, >65535 and non-numbers are warned and ignored, like TCPACKMAX. Carried on AmiSana2If.tcp_grow_rtt (set at open beside RXBUFFER). New getter: ami_sana2_get_tcp_grow_rtt().
• bsd_tcp_window_settle (socket.c) reads it from the socket's own nx_tcp_socket_connect_interface. On the passive path, that is the accepted socket that select.c:133 passes in, whose interface the syncache set (nx_tcp_syncache.c:1300) before the handshake RTT. It is never the listener, whose connect_interface is NULL.
• ami_bsd_tcp_window_settle and ami_bsd_tcp_window_burst_bound both take the threshold (0 = built-in 10).
• The setting is read from the interface file alone and needs no rebuild. It takes effect when the interface is next attached.
• No NetX change, no submodule change.

Hardware leg (2.4 GHz WiFiPi, default vs TCPGROWRTT=2, same head): a connect with a handshake RTT of 2 ms or more must show rwnd > 100352 with TCPGROWRTT=2. If it does not, the key silently fell back to the default.

Tests:

• test_config: the key is optional and parsed; 0, 65536 and fast are each reported once.
• pool_window: per-interface threshold cases for settle and burst_bound.
• sana2_device: the key reaches the interface. The getter is composed with settle the way socket.c does it: a NULL interface keeps rtt >= 10, and TCPGROWRTT=2 grows a 2 ms connect but not a 1 ms one.
• Full host ctest: 453/453 pass.
• Cross build default on playhouse2: bsdsocket.library is 365408 bytes, against 365308 on 08c23b2 (+100 B). The budget of 366000 is unchanged.
• check-stack-frames.sh is clean.

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