fix: log SJIS decoding errors instead of silently discarding them

Add SJISToUTF8Lossy() that wraps SJISToUTF8() and logs decode errors at
slog.Debug level. Replace all 31 call sites across 17 files that previously
discarded the error with `_, _ =`. This makes garbled text from malformed
SJIS client data debuggable without adding noise at default log levels.

docs/technical-debt.md

@@ -102,17 +102,7 @@ Suggested split: `repo_guild.go` (core CRUD), `repo_guild_members.go`, `repo_gui
 
 ~~**a) `fmt.Sprintf` inside structured logger calls (6 sites):**~~ **Fixed.** All 6 sites now use `zap.Uint32`/`zap.Uint8`/`zap.String` structured fields instead of `fmt.Sprintf`.
 
-**b) 20 silently discarded SJIS encoding errors in packet parsing:**
-
-The pattern `m.Field, _ = stringsupport.SJISToUTF8(...)` appears across:
-- `network/binpacket/msg_bin_chat.go:43-44`
-- `network/mhfpacket/msg_mhf_apply_bbs_article.go:33-35`
-- `network/mhfpacket/msg_mhf_send_mail.go:38-39`
-- `network/mhfpacket/msg_mhf_update_guild_message_board.go:41-42,50-51`
-- `server/channelserver/model_character.go:175`
-- And 7+ more packet files
-
-A malformed SJIS string from a client yields an empty string with no log output, making garbled text impossible to debug. The error return should at least be logged at debug level.
+~~**b) 20+ silently discarded SJIS encoding errors in packet parsing:**~~ **Fixed.** All call sites now use `SJISToUTF8Lossy()` which logs decode errors at `slog.Debug` level.
 
 ### ~~6. Inconsistent transaction API~~ (Fixed)
 
@@ -151,5 +141,5 @@ Based on impact and the momentum from recent repo-interface refactoring:
 4. **Split `repo_guild.go`** — last oversized file after the recent refactoring push
 5. ~~**Fix `fmt.Sprintf` in logger calls**~~ — **Done**
 6. ~~**Add `LoopDelay` Viper default**~~ — **Done**
-7. **Log SJIS decoding errors** — improves debuggability for text issues
+7. ~~**Log SJIS decoding errors**~~ — **Done**
 8. ~~**Standardize on `BeginTxx`**~~ — **Done**