security(desktop): IPC serial-command-injection + over-broad shell capability + ADR-178 (ruvnet#1100)

* fix(security): desktop IPC serial-command-injection + over-broad shell capability (ADR-178)

Beyond-SOTA security review of wifi-densepose-desktop (Tauri v2). Two real
findings, each MEASURED on Windows (crate builds + tests under
--no-default-features):

WDP-DESK-01 (MODERATE) — serial command injection via configure_esp32_wifi.
The #[tauri::command] handler concatenated webview-supplied ssid/password into
newline-terminated serial commands with no validation; a \r\n let a compromised
webview inject an arbitrary follow-up firmware command (reboot/erase). Added
validate_wifi_credentials() enforcing WPA2 length bounds and rejecting all
control characters, called fail-closed before any serial write. Pinned by 3
new tests (rejects \r\n / \n / NUL injection, rejects out-of-range, accepts
valid boundaries).

WDP-DESK-02 (MODERATE) — removed unused shell:allow-execute / shell:allow-open
from capabilities/default.json. The Rust backend spawns processes via
std::process::Command (bypassing the allowlist) and the UI only uses
dialog.open; the shell perms were unused privilege granting the webview
arbitrary host command execution on compromise. Regenerated capabilities.json
confirms only core:default + dialog perms remain.

lib tests 18 -> 21 (+3 pins), integration 21 -> 21, 0 failed. Python
deterministic proof unchanged (f8e76f21...46f7a; desktop off the signal path).

Co-Authored-By: claude-flow <ruv@ruv.net>

* docs(adr): ADR-178 — desktop IPC injection fix + capability least-privilege

Records the 2 MEASURED MODERATE fixes in feddcde9d: WDP-DESK-01 (webview
ssid/password \r\n-injected arbitrary firmware serial commands → validated
fail-closed) and WDP-DESK-02 (unused shell:allow-execute/open capability
granted to the webview → removed). 30-command IPC surface + capability scope
audited; 6 dimensions clean-with-evidence. desktop 18→21.

Co-Authored-By: claude-flow <ruv@ruv.net>

Author: ruvnet