fix: the four tiers stop describing features they do not have, and three gates start failing

The reading pass over this repo found the tiers disagreeing with themselves,
and every fix here was measured.

**The turn-around guard is real code.** `tsb_asm` and `tsb_tricks` wrote
`for (std::uint8_t guard = 46; guard; --guard) ;` between taking the one-wire
line and the first UDR0 store, under a comment naming it a turn-around guard.
It has no side effect, so GCC deleted it - `sts UCSR0B` went straight to
`sts UDR0` - while the hand-written oracle spends six bytes on that wait and
libavr's own half-duplex spends them through `delay::cycles`. Two of four
tiers described a feature they did not have, which made the size gradient a
comparison between different loaders. `avr::delay::cycles<one bit time>()`
bottoms out in asm and cannot be deleted.

**The entry belongs to the library, and hand-rolling it was expensive.** Three
tiers wrote their own naked `.vectors` stub with `asm volatile("clr
__zero_reg__")` - which design.md fences to libavr and never a port, and which
`tsb_tricks` denied having in its own title line. `avr::startup::entry` also
keeps the body `noinline` for a stated reason: avr-ld must not shrink a
`.vectors` section, so a loader inlined into one forfeits call relaxation
everywhere. `tsb_pure` came out **836 -> 734** bytes for that alone.
`stack::hardware` - the reset value this part guarantees, with the write kept
where a part does not - saved another four, which is what let `tsb_asm` afford
the guard it had been four bytes short of. It fills its 512-byte section
exactly now, with the whole feature set.

**`tsb_pure` had no receive timeout.** Its `rx()` was `read_blocking()`, so a
silent host wedged the password gate and the command loop forever - the one
fix the oracle's own header lists by name, and one the other three tiers
implement. It is bounded now, and 0-on-silence falls through every compare as
theirs does.

Three gates could pass without proving anything. `sizes.py check-readme`
reported a match when every row's lookup missed; `check_size.cmake` used
`CMAKE_MATCH_1` without checking the match succeeded, which is the guard its
sibling `check_unit.cmake` has and it is the size gate; `check_pi.py` raised
IndexError instead of reporting a position-independence break that changed the
image's length. And `check.sh` spelled the 37-chip list a second time beside
make_presets.py, where a chip added to one and missed in the other is a
silently unbuilt chip - it reads the presets now, and produces the same 37 and
12.

tsbtest.py gains the scenario nothing covered: a wrong password byte must
neither activate the loader nor reach the emergency erase behind it. Red-green
on a tier with the refusal removed.

Smaller, all measured or checked: the signature is `hw::db.signature` in every
tier as the page size and EEPROM end beside it already were; `act_min` derives
from the clock; pureboot.py's `rjmp` helpers refuse a part past rjmp's
4096-word reach rather than silently folding an offset (unreachable today, the
ATtiny85 sits exactly on it); the host tool calls space 2 `data` as the wire
and the loader do; `.clangd` strips the fifth GCC-only flag the build passes;
pbrig's bitclock guard reads its own ladder; pbreloc's unexplained retry is
gone, the write being reliable on five runs without it; and the four tier
sizes live in oracle/README.md's table instead of four file headers and a
CMake comment.

`--poke` before `--peek` turned out to be right - pbtest.py round-trips a poke
through the peek behind it - so the parser order and README say so now.

Every chip green, the README size table matching every image.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

tsb/tsb_pure.cpp

@@ -63,26 +63,48 @@ constexpr std::uint16_t build_date = 26 * 512 + 7 * 32 + 20;
 // The 16-byte device-info block the host reads on activation. A flash_table
 // keeps it in progmem with no .data image (there is no crt to copy one).
 // clang-format off
-inline constexpr std::array<std::uint8_t, 16> info_data = {
+inline constexpr auto info_data = std::to_array<std::uint8_t>({
     'T', 'S', 'B',
     build_date & 0xFF, build_date >> 8,
     0xF3,             // status byte (native-UART fixed-baud lineage)
-    0x1E, 0x95, 0x0F, // ATmega328P signature
+    avr::hw::db.signature[0], avr::hw::db.signature[1], avr::hw::db.signature[2],
     page / 2,         // page size in words
     (app_end / 2) & 0xFF, (app_end / 2) >> 8, // app-flash boundary, words
     eeprom_end & 0xFF, eeprom_end >> 8,
     0xAA, 0xAA, // ATmega processor-type marker (bytes 14 == 15)
-};
+});
 // clang-format on
 using info = avr::flash_table<info_data>;
 
-// Blocking byte read/write over the one-wire line: read() releases the line to
-// the receiver, write() takes it and holds it until the frame is out.
+// The lockout-proof floor for the receive window: the oracle's F_CPU/1MHz, so
+// it follows the clock rather than restating it.
+constexpr auto act_min = static_cast<std::uint8_t>(dev::clock.hz / 1'000'000);
+
+// The receive window, pre-floored where it is set. In .noinit: there is no crt
+// to clear a .bss image, and run() stores it before the first receive.
+[[gnu::section(".noinit")]] std::uint8_t window;
+
+// Bounded byte read over the one-wire line - read() releases the line to the
+// receiver - answering 0 on silence. That 0 falls through every compare below:
+// not a knock, not a confirm, not a command, so a silent host unwinds the
+// loader to the application from anywhere and a mid-session cable pull cannot
+// wedge it. The oracle lists that timeout among its own fixes, and a blocking
+// read is how a tier loses it.
 std::uint8_t rx()
 {
-	return serial.read_blocking();
+	std::uint16_t outer = static_cast<std::uint16_t>(window) << 8;
+	do {
+		std::uint8_t fine = 0;
+		do {
+			if (auto byte = serial.read()) {
+				return *byte;
+			}
+		} while (--fine);
+	} while (--outer);
+	return 0;
 }
 
+// write() takes the line and holds it until the frame is out.
 void tx(std::uint8_t byte)
 {
 	serial.write(byte);
@@ -270,9 +292,11 @@ gate password_gate()
 	avr::init<serial_t>();
 
 	// Activation: the host knocks three '@' inside a window whose length is the
-	// config page's timeout byte (floored so a corrupt page can never lock the
-	// loader out). An idle port times out and boots the application.
-	__uint24 idle = static_cast<__uint24>(avr::flash_load(flash_ptr(app_end + 2)) | 16) << 16;
+	// config page's timeout byte, floored so a corrupt page can never lock the
+	// loader out. An idle port times out and boots the application; the same
+	// window then bounds every receive of the session.
+	window = avr::flash_load(flash_ptr(app_end + 2)) | act_min;
+	__uint24 idle = static_cast<__uint24>(window) << 16;
 	std::uint8_t knocks = 0;
 	while (knocks < 3) {
 		if (auto byte = serial.read()) {
@@ -324,14 +348,6 @@ gate password_gate()
 } // namespace
 } // namespace tsb
 
-// Reset lands here: BOOTRST vectors to the boot section base and .vectors is
-// laid first, so this is the first instruction executed. No crt ran, so set the
-// stack pointer before anything is called.
-extern "C" [[gnu::naked, gnu::used, gnu::section(".vectors")]] void __boot_entry()
-{
-	SP = RAMEND;
-	// The one line of crt this loader needs: compiled code assumes
-	// __zero_reg__ (r1) is 0, and power-on registers are undefined.
-	asm volatile("clr __zero_reg__");
-	tsb::run();
-}
+// Reset lands at the boot section base (BOOTRST): the entry stub in .vectors
+// is laid first and does the one line of crt a crt-less image needs.
+template struct avr::startup::entry<tsb::run, avr::startup::stack::hardware>;