core+gui: compact BAL QR v2 wire format + best-of compression default

BAL QR export now ships a fixed 11-char 'BAL1<TTT><iii><F><payload>' header
(3-char base36 zero-padded count fields, cap 46655 frames) and, by default,
the shorter of plain vs zlib+base64 (flag '0'/'Z'). parse_frame remains dual:
legacy 'BALQR1|total|index|flags|' frames still import. detect_format accepts
'BAL1'. New exports are not readable by builds older than this change.

Also add a will-detail test for heir/willexecutor addresses and decoded
OP_RETURN payloads. Docs updated (CHANGELOG, COMPATIBILITY, PLAN_QR_TRANSFER,
README); 345 core tests + 89 GUI tests pass.

README.md

@@ -94,7 +94,9 @@ exists, then enable it from **Tools → Plugins**.
 From the will list (**Export → QR Codes**) a will can be exported as a
 sequence of QR codes and imported on another device (**Import via QR**). The
 export offers All / Valid / Valid-NC filters plus a QR size preset
-(150–1800 bytes/frame); the import flow reviews and sign each transaction
+(150–1800 bytes/frame) and ships the default **BAL QR** format already
+compressed whenever that is smaller (best-of zlib, flag per frame); the import
+flow reviews and sign each transaction
 one at a time, then proposes exporting the signed transactions. When
 Electrum's `audio_modem` plugin is enabled (optional, requires `amodem` +
 PortAudio) Send/Receive audio buttons complement the QR channel. See