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>

CMakeLists.txt

@@ -84,26 +84,24 @@ endfunction()
 # watchdog bail, one-wire half-duplex, config-page activation timeout, password
 # gate, emergency erase, config/flash/EEPROM read-write. They differ only in how,
 # and the size gradient is the cost of that "how".
-#   tsb_asm   - the tricks tier's C++ with exactly two routines in asm (the
-#                 bounded rx and the page-store loop - the two whose remaining
-#                 cost is the C ABI itself): 510 B in the 512 B section the
-#                 hand-written 500 B oracle occupies. Everything else, from
-#                 bring-up to dispatch, is C++ on libavr.
+#   tsb_asm    - the tricks tier's C++ with exactly two routines in asm: the
+#                 bounded rx and the page-store loop, the two whose remaining
+#                 cost is the C ABI itself. Everything else, bring-up to
+#                 dispatch, is C++ on libavr.
 #   tsb_tricks - no asm at all: the whole-loader register allocation lives in
 #                 global register variables (Y walks the page pointer), every
 #                 helper is a tiny noinline primitive placed by the
 #                 global-register store rules, pages stream straight to
-#                 SPM/EEPROM, and the bring-up is the two reset-non-default
-#                 registers only. 526 B in the 1 KB section (BOOTSZ=10) - 14
-#                 over the oracle's section, from 168 over at this tier's first
-#                 floor.
-#   tsb_pure  - pure idiomatic libavr, one function per command, TU-local
-#                 (internal linkage), streaming (no SRAM page buffer): 836 B in
-#                 the 1 KB section.
+#                 SPM/EEPROM.
+#   tsb_pure   - pure idiomatic libavr, one function per command, TU-local
+#                 (internal linkage), streaming (no SRAM page buffer).
 #   tsb_policy - the policy floor: pureboot's rules (no asm, no register
-#                 variables) with every pureboot lesson applied. 638 B in the
-#                 1 KB section - the measured evidence that the 512 B fit is a
-#                 property of the mechanisms philosophy #5 bans.
+#                 variables) with every pureboot lesson applied, and the
+#                 measured evidence that the 512 B fit is a property of the
+#                 mechanisms philosophy #5 bans.
+#
+# What each measures is oracle/README.md's table, which is the one place the
+# four numbers and the hand-written loader's own are compared.
 #
 # add_tsb_variant(<name> <boot-section-bytes>)
 function(add_tsb_variant name bytes)