tsb: document the three tiers at full parity in the build file

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

CMakeLists.txt

@@ -45,9 +45,13 @@ endif()
 # loader has no use for the crt or the vector table. The naked entry sits in
 # .vectors, laid first, and runs. The boot base is FLASHEND+1 minus the section
 # size; the linker section-start and the source's boot_bytes agree.
-#   tsb_asm   — inline-asm variant, the headline: ≤512 B, the 512 B section.
-#   tsb_pure / tsb_tricks — pure-C++ and compiler-trickery variants, larger,
-#                 shown in the 1 KB section (BOOTSZ=10) they fit.
+# All three implement the full oracle feature set (see oracle/README.md):
+# watchdog bail, one-wire half-duplex, config-page activation timeout, password
+# gate, emergency erase, config/flash/EEPROM read-write. They differ only in how.
+#   tsb_asm   — minimal inline asm, the headline: 502 B in the 512 B section,
+#                 matching the hand-written oracle's size and features.
+#   tsb_tricks — compiler trickery, no asm: 808 B in the 1 KB section (BOOTSZ=10).
+#   tsb_pure  — pure idiomatic libavr: 950 B in the 1 KB section.
 #
 # add_tsb_variant(<name> <boot-section-bytes>)
 function(add_tsb_variant name bytes)