tsb: one spelling of the clock and the baud, and the asm's bits by name

The tricks and asm tiers spelled 16_MHz three times each and now name it once;
the policy tier's lean bring-up solves the baud from the link config the
serial unit is declared with instead of restating 115200 8N1. The asm tier's
immediates - RXEN0, RXC0's bit, SPMEN, PGWRT|SPMEN and the three UART
addresses - are operands read from the chip database rather than numbers with
comments beside them, so avr/io.h is left for SPMCSR's I/O address alone; pure
and tricks drop an include nothing used. Headers that described the code as it
was are corrected (pure's "only attribute", the policy tier's history), the
rule references leave the comments, and locals are const auto.

All four images are byte-identical to before, in both modes for tsb_asm.

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

tsb/tsb_tricks.cpp

@@ -31,8 +31,6 @@
 
 #include <libavr/libavr.hpp>
 
-#include <avr/io.h> // SP / RAMEND for the crt-free boot entry
-
 using namespace avr::literals;
 namespace spm = avr::spm;
 namespace ee = avr::eeprom;
@@ -55,6 +53,8 @@ constexpr std::uint8_t confirm = '!';
 constexpr std::uint8_t request = '?';
 constexpr std::uint8_t knock = '@';
 
+constexpr auto clock = 16_MHz;
+
 // Boot geometry for the 1 KB boot section (BOOTSZ=10); the page size and the
 // flash/EEPROM extents are the chip database's to know. app_end is the config
 // page (TSB's LASTPAGE), one page below the boot section.
@@ -64,20 +64,20 @@ constexpr std::uint16_t app_end = spm::flash_bytes - boot_bytes - page;
 constexpr std::uint16_t eeprom_end = avr::hw::db.mem.eeprom_size - 1;
 
 // Lockout-proof floor for the activation window: the oracle's F_CPU/1MHz, so
-// it follows the clock rather than restating it (rule 41).
-constexpr auto act_min = static_cast<std::uint8_t>((16_MHz).hz / 1'000'000);
+// it follows the clock rather than restating it.
+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 build_date = 26 * 512 + 7 * 32 + 20;
 
 // Fixed 115200 8N1; the library solves UBRR + U2X from clock and baud.
-constexpr auto baud = avr::uart::solve_baud(16_MHz, 115200_Bd, 8, avr::uart::parity::none);
+constexpr auto baud = avr::uart::solve_baud(clock, 115200_Bd, 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. Derived from the solved rate, so it follows
 // the link rather than a count measured against one.
-constexpr auto guard_cycles = static_cast<std::uint32_t>((16_MHz).hz / baud.actual);
+constexpr auto guard_cycles = static_cast<std::uint32_t>(clock.hz / baud.actual);
 
 // The 16-byte device-info block, streamed out on activation.
 // clang-format off
@@ -160,7 +160,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 // One flash byte <- [g_addr++] (the advance right before ret - see header).
 [[gnu::noinline, gnu::noclone]] std::uint8_t sflash()
 {
-	std::uint8_t byte = avr::flash_load(flash_ptr(g_addr));
+	const auto byte = avr::flash_load(flash_ptr(g_addr));
 	++g_addr;
 	return byte;
 }
@@ -168,7 +168,7 @@ const std::uint8_t *flash_ptr(std::uint16_t addr)
 // One EEPROM byte <- [g_addr++].
 [[gnu::noinline, gnu::noclone]] std::uint8_t eerd()
 {
-	std::uint8_t byte = ee::read<no_spm>(g_addr);
+	const auto byte = ee::read<no_spm>(g_addr);
 	++g_addr;
 	return byte;
 }
@@ -263,7 +263,7 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 	g_receiving = 0;
 
 	// Activation: 3x'@', each inside the config page's timeout window (rx
-	// floors it so a corrupt page cannot lock the loader out); anything else  -
+	// floors it so a corrupt page cannot lock the loader out); anything else -
 	// including silence - hands over.
 	g_window = avr::flash_load(flash_ptr(app_end + 2));
 	for (std::uint8_t k = 3; k; --k) {
@@ -289,7 +289,7 @@ extern "C" [[noreturn]] void tsb_app(); // the application's reset vector: --def
 			sendf();
 			break;
 		}
-		std::uint8_t got = rx();
+		const auto got = rx();
 		if (got == 0) {
 			if (mask == 0) {
 				continue;