tsb: the protocol the tiers share is written once, and its bytes by name

tsb/tsb.hpp holds the link, the handshake and command bytes, the guard
policy and SPM interlock, the boot geometry, act_min/comm_window and a
consteval builder for the 16-byte info block, whose BUILDSTATE and ATmega
marker bytes are now named rather than commented. Every tier's image is
byte-identical to before.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

tsb/tsb.hpp

@@ -0,0 +1,106 @@
+// The TinySafeBoot native-UART fixed-baud protocol, as every tier speaks it:
+// the link, the handshake and command bytes, the boot geometry and the
+// device-info block the host reads on activation.
+
+#pragma once
+
+#include <libavr/libavr.hpp>
+
+namespace tsb {
+
+using namespace avr::literals;
+namespace spm = avr::spm;
+namespace ee = avr::eeprom;
+
+constexpr auto clock = 16_MHz;
+constexpr auto baud_rate = 115200_Bd;
+
+// Fixed 8N1; the library solves UBRR + U2X from clock and baud.
+constexpr auto baud = avr::uart::solve_baud(clock, baud_rate, 8, avr::uart::parity::none);
+
+// One bit time on the wire: the turn-around a shared-line peer needs to stop
+// driving before this one starts.
+constexpr auto guard_cycles = static_cast<std::uint32_t>(clock.hz / baud.actual);
+
+// The loader is purely polled - it never enables interrupts - so every SPM and
+// EEPROM lock folds to nothing under this posture.
+constexpr auto off = avr::irq::guard_policy::unused;
+
+// Strict request/response: every SPM operation is waited out before the next
+// byte moves, so no flash operation is ever in flight at an EEPROM access -
+// the write procedure's step 2 has nothing to guard, the omission the
+// datasheet grants (DS40002061B section 8.6.3).
+constexpr auto no_spm = ee::spm_interlock::omitted;
+
+constexpr std::uint8_t confirm = '!';
+constexpr std::uint8_t request = '?';
+constexpr std::uint8_t knock = '@';
+constexpr std::uint8_t activation_knocks = 3;
+
+namespace command {
+constexpr std::uint8_t read_flash = 'f';
+constexpr std::uint8_t write_flash = 'F';
+constexpr std::uint8_t read_eeprom = 'e';
+constexpr std::uint8_t write_eeprom = 'E';
+constexpr std::uint8_t read_config = 'c';
+constexpr std::uint8_t write_config = 'C';
+} // namespace command
+
+// The lockout-proof floor for the receive window: the oracle's F_CPU/1MHz.
+constexpr auto act_min = static_cast<std::uint8_t>(clock.hz / 1'000'000);
+// Post-activation window: the host gets seconds, not milliseconds, mid-session.
+constexpr std::uint8_t comm_window = 200;
+
+constexpr std::uint16_t page = spm::page_bytes;
+constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
+
+// The config page (TSB's LASTPAGE), one page below the boot section: the
+// application's jump vector, then these.
+consteval std::uint16_t config_page(std::uint16_t boot_bytes)
+{
+	return spm::flash_bytes - boot_bytes - page;
+}
+constexpr std::uint8_t timeout_at = 2;
+constexpr std::uint8_t password_at = 3;
+
+// The firmware version stamp, in the encoding the host decodes.
+consteval std::uint16_t date_stamp(std::uint16_t year, std::uint16_t month, std::uint16_t day)
+{
+	return year * 512 + month * 32 + day;
+}
+
+// BUILDSTATE F3: the native-UART fixed-baud lineage.
+constexpr std::uint8_t build_state = 0xF3;
+constexpr std::uint8_t atmega_marker = 0xAA;
+
+consteval auto info_block(std::uint16_t build_date, std::uint16_t app_end)
+{
+	const auto low = [](std::uint16_t word) { return static_cast<std::uint8_t>(word); };
+	const auto high = [](std::uint16_t word) { return static_cast<std::uint8_t>(word >> 8); };
+	const std::uint16_t app_end_words = app_end / 2;
+	return std::to_array<std::uint8_t>({
+	    'T',
+	    'S',
+	    'B',
+	    low(build_date),
+	    high(build_date),
+	    build_state,
+	    avr::hw::db.signature[0],
+	    avr::hw::db.signature[1],
+	    avr::hw::db.signature[2],
+	    page / 2,
+	    low(app_end_words),
+	    high(app_end_words),
+	    low(eeprom_end),
+	    high(eeprom_end),
+	    atmega_marker,
+	    atmega_marker,
+	});
+}
+
+inline const std::uint8_t *flash_ptr(std::uint16_t addr)
+{
+	return reinterpret_cast<const std::uint8_t *>(addr);
+}
+
+} // namespace tsb