docs: Compress the descriptor QR

The watch-only wallet file from exportwatchonlywallet does not fit in a
QR even compressed (12,288 bytes; 2,365 gzipped against qr's 2,331-byte
limit), so the guide sends the public descriptors. gzip shrinks them
from 1,975 to 619 bytes, which takes qr's code from version 37 to 19,
about half the width, and makes the scan more reliable.

The online side reads the QR with zbarcam -Sbinary and pipes it through
gunzip. Without -Sbinary, ZBar rewrites the bytes as text and gunzip
fails. qrencode needs -8 for the same data; without it the input stops
at the first zero byte.

Validation: on two separate Bitcoin Core 32.0rc2 regtest nodes with the
BIP-93 test vector, the gzipped listdescriptors output went through qr
and back through zbarimg with the guide's flags and gunzip, all 8
descriptors imported with success, and both wallets gave the same first
address. qrencode -8 round-tripped the same bytes. Docs-only change.

Refs #92

Claude-Session: https://claude.ai/code/session_01T233rKgZqE5wzDm3EVTHL1

Author: claude

docs/user/guide.md

@@ -216,24 +216,24 @@ are present.
 ### 2. Create the online watch-only wallet
 
 The tutorial moves a watch-only wallet file made by `exportwatchonlywallet`.
-That file is far too large for a QR, so send the public descriptors instead.
-On the offline computer:
+That file does not fit in a QR even compressed, so send the public descriptors
+instead, compressed with `gzip`. On the offline computer:
 
 ```bash
-bitcoin-cli -rpcwallet=offline_wallet listdescriptors | jq -cj '[.descriptors[] | {desc,timestamp,active,internal,range,next_index}]' | qr
+bitcoin-cli -rpcwallet=offline_wallet listdescriptors | jq -cj '[.descriptors[] | {desc,timestamp,active,internal,range,next_index}]' | gzip -9 | qr
 ```
 
-The QR holds about 2,000 characters; see [QR troubleshooting](#qr-troubleshooting)
-if it does not fit. Public descriptors cannot spend, but they reveal wallet
-activity. Do not use a website, cloud scanner, chat service, or synced
-clipboard.
+The compressed descriptors take about 620 bytes; see
+[QR troubleshooting](#qr-troubleshooting) if the QR does not fit. Public
+descriptors cannot spend, but they reveal wallet activity. Do not use a
+website, cloud scanner, chat service, or synced clipboard.
 
-On the online computer, create a blank watch-only wallet and scan the QR into
-it:
+On the online computer, create a blank watch-only wallet, then scan and
+decompress the QR into it:
 
 ```bash
 bitcoin-cli -named createwallet wallet_name=watch_only_wallet disable_private_keys=true blank=true
-zbarcam --raw --oneshot -Sdisable -Sqrcode.enable |
+zbarcam --raw --oneshot -Sdisable -Sqrcode.enable -Sbinary | gunzip |
   bitcoin-cli -rpcwallet=watch_only_wallet -stdin importdescriptors
 ```
 
@@ -372,7 +372,9 @@ arbitrary-HRP format direction is not yet merged into that specification.
 
 Maximize the terminal and reduce its font size if a QR does not fit. Keep `qr`
 connected to the terminal; redirecting its output creates an image file. If `qr`
-is unavailable, `qrencode -t ANSIUTF8` shows the same QR. Only public
+is unavailable, `qrencode -8 -t ANSIUTF8` shows the same QR. Reading the
+compressed descriptors needs ZBar 0.23.1 or newer for `-Sbinary`; without it,
+ZBar rewrites the bytes as text and `gunzip` fails. Only public
 descriptors, xpubs, PSBTs, and signed transactions may cross the offline
 boundary by QR.