feat(docs): Add footprint info

[david-cermak]

Tested on ESP32-S3, IDF v5.5, server example with Ed25519 host key.

Flash Size (idf.py size-components)

Binary size: 1,057 KB (31% of partition free)

Key components:

Archive	Total	Flash Code	Flash Data (.rodata)	DRAM (.data+.bss)
liblibssh.a	233 KB	101 KB	117 KB	10 KB
libmbedcrypto.a	78 KB	72 KB	6 KB	< 1 KB
libmain.a (app)	5 KB	3 KB	< 1 KB	2 KB
libpthread.a	1 KB	1 KB	< 1 KB	< 1 KB

For reference, largest non-SSH components: libnet80211.a (WiFi, 143 KB), liblwip.a (72 KB),
libc.a (67 KB), libwpa_supplicant.a (64 KB).

Runtime Memory (Heap)

Heap consumption during an SSH session lifecycle:

Stage	Free Heap	Delta	Min Free Ever
Before ssh_init()	257,564	—	254,784
After ssh_init()	254,996	−2.6 KB	254,784
After ssh_bind_accept()	249,508	−5.5 KB	247,860
Session ready (auth + channel)	229,644	−19.9 KB	225,672

• Total heap cost of one SSH session: ~28 KB
• Transient peak: ~32 KB (min-ever 225,672 — crypto buffers during key exchange are freed afterward)
• ssh_init() itself is cheap (~2.6 KB); the key exchange + session setup is the expensive part (~20 KB)

Runtime Memory (Stack)

Main task stack (CONFIG_ESP_MAIN_TASK_STACK_SIZE=8192):

Stage	Stack HWM (bytes free)	Used
Before ssh_init()	5,508	~2.7 KB
After ssh_bind_accept()	4,884	~3.3 KB
Session ready (auth + channel)	3,460	~4.7 KB

Peak stack usage is ~4.7 KB, leaving ~3.5 KB headroom with the default 8 KB stack.

Task List (during active session)

No additional tasks are spawned by libssh or mbedtls. All work runs in the main task.
CONFIG_MBEDTLS_THREADING_PTHREAD=y only adds mutex protection — it does not create threads.

Task	Description	Stack HWM	Prio
main	SSH server (application)	3,460	1
IDLE0 / IDLE1	FreeRTOS idle (dual-core)	~680	0
tiT	lwIP TCP/IP	1,412	18
Tmr Svc	FreeRTOS timer service	1,308	1
ipc0 / ipc1	Inter-processor comms	~520	24
sys_evt	ESP-IDF event loop	588	20
esp_timer	High-resolution timer	3,076	22
wifi	WiFi driver	3,368	23