feat(seams): overlay-registered QEMU-Linux board kinds + backend profile registry

registerPiFamilyKind + ProBoardDef.piFamily route an overlay board through
the existing Raspberry Pi bridge path (WebSocket qemu, VFS panel, boot
terminal); register_pi_board_profile lets the overlay add the matching
PI_CONFIGS entry (cpu/image_set/...) at registration time.

backend/app/services/qemu_manager.py

@@ -148,6 +148,31 @@ PI_CONFIGS: dict[str, dict] = {
 # that still send the legacy "start_pi" message without a board field.
 DEFAULT_PI_BOARD = 'raspberry-pi-3'
 
+# Every key a profile must carry — the boot path reads exactly these.
+_PI_PROFILE_KEYS = frozenset(
+    {'qemu', 'cpu', 'smp', 'memory', 'image_set', 'kernel', 'initramfs',
+     'rootfs', 'bus'}
+)
+
+
+def register_pi_board_profile(board_type: str, cfg: dict) -> None:
+    """Register (or override) a QEMU-Linux board profile at runtime.
+
+    Seam for private overlays to add board profiles (same shape as the
+    ``PI_CONFIGS`` entries) without the OSS tree carrying their names —
+    e.g. an ARM64 SBC that boots the generic arm64 image set with a
+    different -cpu model. Must be called before the board's first
+    ``start_instance`` (overlay registration time is fine).
+    """
+    missing = _PI_PROFILE_KEYS - cfg.keys()
+    if missing:
+        raise ValueError(
+            f'pi board profile {board_type!r} missing keys: {sorted(missing)}'
+        )
+    PI_CONFIGS[board_type] = dict(cfg)
+    logger.info('registered QEMU board profile %r (cpu=%s, image_set=%s)',
+                board_type, cfg['cpu'], cfg['image_set'])
+
 
 # ── Pluggable I2C/SPI/UART dispatcher ────────────────────────────────────
 #

frontend/src/lib/proBoardRegistry.ts

@@ -21,6 +21,7 @@ import {
   BOARD_KIND_LABELS,
   BOARD_KIND_FQBN,
   BOARD_SUPPORTS_MICROPYTHON,
+  registerPiFamilyKind,
   type BoardKind,
 } from '../types/board';
 
@@ -54,6 +55,10 @@ export interface ProBoardDef {
    *  through the ESP32 bridge path and picks the machine/engine type. Omit for
    *  boards that provide createSimulator (RP2350 class) or AVR/RP2040. */
   esp32Family?: 'esp32' | 'esp32-s3' | 'esp32-c3' | 'esp32-c6';
+  /** QEMU-Linux run-path routing: route this kind through the Raspberry Pi
+   *  bridge (backend qemu WebSocket, VFS panel, boot terminal). The overlay
+   *  must also register a matching backend profile for the kind. */
+  piFamily?: boolean;
   /** Canvas renderer. Receives the placed board's props; return a React node.
    *  When omitted, the canvas renders `<tag id=... style=absolute@x,y>`. */
   render?: (props: { id: string; x: number; y: number; running: boolean }) => React.ReactNode;
@@ -120,6 +125,7 @@ export function registerProBoards(defs: ProBoardDef[]): void {
     (BOARD_KIND_LABELS as Record<string, string>)[kind] = def.label;
     (BOARD_KIND_FQBN as Record<string, string | null>)[kind] = def.fqbn;
     if (def.supportsMicroPython) BOARD_SUPPORTS_MICROPYTHON.add(kind);
+    if (def.piFamily) registerPiFamilyKind(def.kind);
   }
   version++;
   for (const l of listeners) l();

frontend/src/types/board.ts

@@ -32,11 +32,24 @@ export type BoardKind =
 
 export type LanguageMode = 'arduino' | 'micropython' | 'espidf';
 
-/** True for every Raspberry Pi backed by the QEMU bridge (Zero, 1, 2, 3, 4, 5).
+/** Extra QEMU-Linux board kinds registered at runtime by a private overlay
+ *  (proBoardRegistry defs with `piFamily: true`). They route through the same
+ *  backend qemu bridge, VFS panel and terminal UX as the Raspberry Pi family. */
+const PI_FAMILY_EXTRA_KINDS = new Set<string>();
+
+export function registerPiFamilyKind(kind: string): void {
+  PI_FAMILY_EXTRA_KINDS.add(kind);
+}
+
+/** True for every Raspberry Pi backed by the QEMU bridge (Zero, 1, 2, 3, 4, 5)
+ *  and any overlay-registered QEMU-Linux board (registerPiFamilyKind).
  *  Excludes the Pico boards (RP2040, browser emulation). */
 export function isPiBoardKind(kind: BoardKind | string): boolean {
-  return typeof kind === 'string' && kind.startsWith('raspberry-pi-')
-    && kind !== 'raspberry-pi-pico';
+  if (typeof kind !== 'string') return false;
+  return (
+    (kind.startsWith('raspberry-pi-') && kind !== 'raspberry-pi-pico') ||
+    PI_FAMILY_EXTRA_KINDS.has(kind)
+  );
 }
 
 /** True for STM32 boards backed by the QEMU bridge (libqemu-arm via