Replace periph.io with direct i2c-dev access; make the module dependency-free

[cafedomingo]

The driver's entire use of periph.io reduces to: open /dev/i2c-1, latch a fixed slave address, write. The kernel's i2c-dev interface gives exactly that with one I2C_SLAVE ioctl plus write(2) — the display protocol never reads, never scans the bus, never changes address. Meanwhile host.Init() registered GPIO/SPI/sysfs drivers this project never touches.

For a binary users curl | sudo bash onto their machines and run as a long-lived service, every dependency is supply-chain surface to trust, update, and audit. This takes the module from three dependencies to zero: the entire codebase is now this repo plus the Go standard library. The project already does raw ioctl work for the VideoCore mailbox in sysinfo, so this doesn't introduce a new style.

Notes for review:

• The bytes on the wire should be identical to what periph produced. Periph's Linux path uses I2C_RDWR for Tx(w, nil) rather than I2C_SLAVE + write(2), but for a single write-only transaction to one fixed address the two are wire-equivalent. A smoke test on real hardware before the next release is still prudent.
• The new tests exist because removing periph meant putting the bus behind a small internal interface anyway — which made the command framing, panel y-offset, and 160-byte burst limit testable for the first time. Those invariants were previously enforced only by trying it on hardware.
• Independent of Verify release checksums and run the display service unprivileged #61/Make install.sh repair partial installs and surface required reboots #62/Fix Pi 5 I2C speed check, stale network stats, and screenshot palette overflow #63; no shared files.