power.rack: pod-hosted TFTP staging client

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Summary

Adds RackController.tftp_put / tftp_delete / tftp_clear / tftp_list — the host-side wrappers for the pod's /tftp/* endpoints. Lets defib stage firmware bytes into the pod's PSRAM, then have the camera's U-Boot fetch from the pod itself (192.168.1.1:69) over the local camera-side LAN.

Why it matters: defib's existing U-Boot+TFTP path requires a host-side TFTP server on port 69 — conflicts with dnsmasq, needs sudo, needs NIC IP plumbing on the camera's subnet. Pod-hosted TFTP gets rid of every one of those: zero host setup, the pod talks to the camera on its own local LAN.

End-to-end measurement

OpenIPC hi3516ev300-nor-neo install (kernel 2.0 MB + rootfs 4.2 MB):

#	Method	Total	Stage	Flash	Host setup
1	Agent write_flash	92 s	—	77 s	none
2	Host TFTP + sf write	132 s	—	100 s	sudo + dnsmasq + NIC IP
3	Pod TFTP + sf write	193 s	50 s	96 s	none

Pod TFTP is slower than host TFTP because the bytes traverse WiFi twice (host upload + pod serve) instead of once (camera-pulls-through-NAPT). It wins on operational simplicity vs Method 2 (no sudo, no port-69 conflict, no NIC plumbing) and is the natural fallback when the agent path isn't usable for a given SoC.

The PSRAM cap on the prototype N8R8 is 8 MB, comfortably holding both kernel + rootfs staged simultaneously.

Companion firmware change

Local-only branch on the rack repo (not on GitHub per project policy): adds tftpd.c — a tiny RFC 1350 TFTP server bound to the W5500 interface, plus POST /tftp/<name> HTTP staging endpoints. Stores files in PSRAM via zero-copy ownership transfer (tftpd_add_file_owned) to avoid the double-buffer OOM that the initial naïve impl hit on a 4 MB rootfs.

Tests

6 new TestTftpStaging cases:

• tftp_put posts correct URL and body
• tftp_put rejects path-traversal / empty names
• tftp_delete one file
• tftp_clear all
• tftp_list returns the JSON dict
• 503 OOM from the pod surfaces as PowerControllerError verbatim (callers can fall back)

Test plan

• uv run pytest tests/ -x -v --ignore=tests/fuzz — 486 passed / 2 skipped
• uv run ruff check tests/ src/defib/
• uv run mypy src/defib/power/rack.py --ignore-missing-imports

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