tsb: vendor the fixed-baud assembly oracle as the size/feature bar

The Seed Robotics native-UART fixed-baud TinySafeBoot (GPLv3), reference
only — not built. Assembles to 500 B with the full feature set, proving
≤512 B and full feature parity are simultaneously reachable. Also drops the
stale empty stk500v2/ leftover.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

oracle/README.md

@@ -0,0 +1,47 @@
+# Oracle — the hand-written TinySafeBoot assembly
+
+`tsb-fixedbaud.asm` is the reference implementation this port is measured
+against: the **native-UART, fixed-baud** TinySafeBoot bootloader, hand-written
+in AVR assembly. It is the size-and-feature bar for the port's `tsb_asm` tier.
+
+- **Source**: <https://github.com/seedrobotics/tinysafeboot>
+  (`firmware_ASM/latest_stable_release/20200727-fixedbaud/main.asm`), the Seed
+  Robotics fixed-baud fork of Julien Thomas' TinySafeBoot.
+- **License**: GPLv3 (see the header in the file). It is vendored here **only as
+  a reference oracle** — it is not compiled, linked, or distributed as part of
+  the MIT-licensed port. Mere aggregation.
+
+## Why this variant
+
+The user chose the fixed-baud, hardware-UART variant deliberately: it is the one
+whose feature set the port must match. It fits the **complete** TSB feature set
+into the 512-byte ATmega boot section:
+
+| Feature | Oracle routine |
+|---|---|
+| Watchdog-reset bail straight to the app | `RESET` (WDRF check) |
+| One-wire half-duplex (RX/TX shorted): RXEN/TXEN toggled per direction, TX turnaround guard | `SetRX` / `SetTX` / `TransmitByte` |
+| Activation timeout read from the config page, with a lockout-proof minimum | `WRX1To` (uses `utimeoutH`) |
+| 3×`@` activation knock | `ActCharRcvd` |
+| Password gate; wrong byte hangs (still draining the UART) | `CheckPassword` |
+| Emergency erase on password `\0` + double-confirm — wipes flash, EEPROM and the config page | `EmergencyErase` |
+| Device-info block (16 bytes) | `SendDeviceInfo` / `DEVICEINFO` |
+| App-flash read/write (`f`/`F`), EEPROM read/write (`e`/`E`), config read/write (`c`/`C`) | `CheckCommands` |
+
+## Assembled size (the bar)
+
+Assembled for the ATmega328P with `avra`:
+
+```
+avra -I /usr/share/avra tsb-fixedbaud.asm   # after uncommenting .include "m328Pdef.inc"
+#   Code : 250 words (500 bytes)  — the whole loader, all features, in the 512 B section
+```
+
+**500 bytes with every feature** — the proof that ≤512 B and full feature parity
+are simultaneously reachable. The port's `tsb_asm` tier matches this bar; `tsb_pure`
+and `tsb_tricks` implement the same protocol at larger sizes in the 1 KB section,
+trading bytes for readability.
+
+The oracle targets 20 MHz / 33333 baud; the port targets 16 MHz / 115200 baud
+(what the simavr protocol test drives). Baud and geometry differ, code size and
+feature set do not.