fix: store and execute multiline commands in pos bank

bank.env is line-oriented (name|description|command) so commands with
real newlines spanned records: bank_load truncated the command to its
first line and the remaining script lines became bogus entries. The
bank_get+cut -f3 retrieval path also truncated at embedded tabs.

- lib/bank-lib.sh: v2 format escapes backslash->\\ and newline->\\n
  in the command field, writes # BANK_VERSION: 2; bank_load decodes
  with printf %b only for v2 files, so existing v1 files load raw
  (backward compatible, verified against the real ts-google entry).
- bin/pos-bank: cmd_show/cmd_run/cmd_edit now read fields from the
  BANK_* arrays via bank_find instead of bank_get+cut.
- tests/t-bank.sh: +13 checks (71 total) - multiline round-trip exact
  bytes, literal backslash-n, v1 raw-backslash compat, v1+re-save
  byte-identical, CLI show/run full script.

bin/pos-bank

@@ -114,12 +114,11 @@ cmd_show() {
     [ -n "${1:-}" ] || err "Usage: pos bank show <name>"
     local name="$1"
     bank_load
-    local entry
-    entry="$(bank_get "$name")" || err "Command not found: $name"
+    bank_find "$name" >/dev/null 2>&1 || err "Command not found: $name"
     local cmd_name cmd_desc cmd_cmd
-    cmd_name="$(printf '%s' "$entry" | cut -f1)"
-    cmd_desc="$(printf '%s' "$entry" | cut -f2)"
-    cmd_cmd="$(printf '%s' "$entry" | cut -f3)"
+    cmd_name="${BANK_NAMES[$BANK_IDX]}"
+    cmd_desc="${BANK_DESCS[$BANK_IDX]}"
+    cmd_cmd="${BANK_CMDS[$BANK_IDX]}"
     echo
     echo "Name: $cmd_name"
     echo
@@ -162,12 +161,11 @@ cmd_run() {
         done
     fi
     bank_load
-    local entry
-    entry="$(bank_get "$name")" || err "Command not found: $name"
+    bank_find "$name" >/dev/null 2>&1 || err "Command not found: $name"
     local cmd_name cmd_desc cmd_cmd
-    cmd_name="$(printf '%s' "$entry" | cut -f1)"
-    cmd_desc="$(printf '%s' "$entry" | cut -f2)"
-    cmd_cmd="$(printf '%s' "$entry" | cut -f3)"
+    cmd_name="${BANK_NAMES[$BANK_IDX]}"
+    cmd_desc="${BANK_DESCS[$BANK_IDX]}"
+    cmd_cmd="${BANK_CMDS[$BANK_IDX]}"
     # Detect parameters
     local params_str
     params_str="$(_extract_params "$cmd_cmd")"
@@ -218,12 +216,11 @@ cmd_edit() {
         name="$(_pick_command)" || return 0
     fi
     bank_load
-    local entry
-    entry="$(bank_get "$name")" || err "Command not found: $name"
+    bank_find "$name" >/dev/null 2>&1 || err "Command not found: $name"
     local old_name old_desc old_cmd
-    old_name="$(printf '%s' "$entry" | cut -f1)"
-    old_desc="$(printf '%s' "$entry" | cut -f2)"
-    old_cmd="$(printf '%s' "$entry" | cut -f3)"
+    old_name="${BANK_NAMES[$BANK_IDX]}"
+    old_desc="${BANK_DESCS[$BANK_IDX]}"
+    old_cmd="${BANK_CMDS[$BANK_IDX]}"
     echo
     log "Editing: $old_name"
     echo