Out-of-bounds read in coredump shell when printing stored-dump target code

In subsys/debug/coredump/coredump_shell.c, print_coredump_hdr() used the 16-bit tgt_code field of a stored Zephyr coredump header directly as an index into coredump_target_code2str[], a fixed 7-element array of string pointers, with no bounds check. A stored coredump whose tgt_code is >= 7 causes an out-of-bounds read of a char* up to ~64K entries past the array; that value is passed as the %s argument to shell_print, which dereferences and walks it as a string. The result is either disclosure of device memory contents to the shell user or a crash when the out-of-bounds pointer is unmapped. The defect is reached via the coredump print shell command (cmd_coredump_print_stored_dump -> pretty_print_coredump -> parse_and_print_coredump -> print_coredump_hdr). The tgt_code field is device-generated and in-range during normal crash handling, so triggering requires local shell access plus the ability to stage or corrupt the stored coredump in the flash/in-memory backend. Introduced in v4.2.0 (commit 13abd7fe730) and present through v4.4.0; fixed by clamping out-of-range codes to the 'unknown' (index 0) entry.