tsb: refactor the pure tier onto libavr sugar

The showcase tier now leans on the helpers it fed back instead of reaching under
them: the info block is an avr::flash_table (no raw [[gnu::progmem]]), a page is
filled with spm::fill(addr, span) (no hand-packed lo|hi<<8 loop), and the
WDT-reset bail reads field<"MCUSR","WDRF">::test() (no read() & {}(1).value).

Zero-overhead throughout: .text stays 740 B, byte-identical across generated and
reflect modes, protocol test green. The info block streams through the existing
address-based send_flash rather than a range-for over the flash_table — the
range-for is a distinct loop that cannot share the loader's one flash streamer,
so it would add 14 B for no functional gain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tsb/tsb_pure.cpp

@@ -4,9 +4,9 @@
 // reimplementing the TinySafeBoot wire protocol (native-UART fixed-baud
 // lineage) on libavr. This variant is written for clarity: well-factored
 // functions, no compiler-specific size hacks, no inline assembly. The only
-// attributes are the ones the task inherently needs — [[gnu::progmem]] for the
+// attribute is the one the task inherently needs — the naked reset entry that
-// flash-resident info block and the naked reset entry that stands in for the
+// stands in for the absent C runtime; the flash-resident info block is a libavr
-// absent C runtime.
+// flash_table.
 //
 // The structure follows the hand-written reference: one SRAM page buffer that
 // every page transfer shares, separate flash/EEPROM leaf routines (so nothing
@@ -48,10 +48,11 @@ constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
 // Firmware version stamp: YY*512 + MM*32 + DD, the encoding the host decodes.
 constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 19;
 
-// The 16-byte device-info block the host reads on activation. Flash-resident so
+// The 16-byte device-info block the host reads on activation. A flash_table
-// it needs no .data (there is no crt to copy it).
+// keeps it in progmem with no .data image (there is no crt to copy one) and
+// reads it back through LPM.
 // clang-format off
-[[gnu::progmem]] constexpr std::uint8_t info[16] = {
+inline constexpr std::array<std::uint8_t, 16> info_data = {
     'T', 'S', 'B',
     build_date & 0xFF, build_date >> 8,
     0xF3,             // status byte (native-UART fixed-baud lineage)
@@ -62,6 +63,7 @@ constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 19;
     0xAA, 0xAA, // ATmega processor-type marker (bytes 14 == 15)
 };
 // clang-format on
+using info = avr::flash_table<info_data>;
 
 // One page staged in SRAM. Scratch that is always filled before it is read, so
 // it lives in .noinit — no startup clear (there is no crt to run one) and no
@@ -116,8 +118,7 @@ bool request_confirm()
 // high, as the SPM word buffer wants).
 void write_flash_page(std::uint16_t addr)
 {
-	for (std::uint16_t i = 0; i < page; i += 2)
+	spm::fill<off>(addr, std::span<const std::uint8_t>{buffer, page});
-		spm::fill<off>(addr + i, static_cast<std::uint16_t>(buffer[i] | (buffer[i + 1] << 8)));
 	spm::write_page<off>(addr);
 	spm::wait();
 }
@@ -200,7 +201,7 @@ void write_config()
 {
 	// A bootloader may be entered by a watchdog reset; the reference loader
 	// hands straight back to the application in that case rather than run.
-	if (avr::hw::reg<"MCUSR">::read() & avr::hw::field<"MCUSR", "WDRF">{}(1).value)
+	if (avr::hw::field<"MCUSR", "WDRF">::test())
 		appjump();
 
 	avr::init<serial_t>();
@@ -225,7 +226,7 @@ void write_config()
 			for (;;) {
 			}
 
-	send_flash(reinterpret_cast<std::uint16_t>(&info[0]), sizeof(info));
+	send_flash(reinterpret_cast<std::uint16_t>(info::storage.data()), info::size());
 
 	for (;;) {
 		tx(confirm); // Mainloop ready