feat(docs): Add footprint info

libssh/footprint.md

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+# libssh ESP-IDF Footprint
+
+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 |