refactor: reorganize client transport and extension internals

- Split client, native, remote, serve, markdown, and SDK internals into focused packages with direct imports.
- Move local and remote transport framing/protocol helpers behind clearer module boundaries.
- Break up the extension injected DOM logic into a separate content dispatch bundle and dedicated content modules.
- Add explicit client handling for passive remote discovery without noisy PQ warnings.
- Keep behavior covered with updated unit, integration, and extension tests.

browser_cli/client/targets.py

@@ -0,0 +1,95 @@
+"""Browser target discovery and local socket resolution."""
+import os
+import socket
+from dataclasses import dataclass
+from pathlib import Path
+
+from browser_cli.endpoints import display_browser_name
+from browser_cli.errors import BrowserNotConnected
+from browser_cli.platform import endpoint_for_alias, is_windows, registry_path
+from browser_cli.registry import load_registry
+
+REGISTRY_PATH = registry_path()
+
+@dataclass(frozen=True)
+class BrowserTarget:
+  profile: str
+  display_name: str
+  socket_path: str
+  remote: str | None = None
+
+def is_reachable_unix_endpoint(endpoint: str) -> bool:
+  """Return True when a Unix socket path exists and accepts connections."""
+  path = Path(endpoint)
+  if not path.exists():
+    return False
+  try:
+    with socket.socket(socket.AF_UNIX, socket.SOCK_STREAM) as sock:
+      sock.settimeout(0.2)
+      sock.connect(endpoint)
+    return True
+  except OSError:
+    return False
+
+def active_endpoints(reg: dict) -> dict:
+  """Return only entries whose endpoint appears reachable."""
+  if is_windows():
+    return dict(reg)
+  return {k: v for k, v in reg.items() if is_reachable_unix_endpoint(v)}
+
+def active_local_browser_targets() -> list[BrowserTarget]:
+  if not REGISTRY_PATH.exists():
+    return []
+  reg = load_registry(REGISTRY_PATH)
+  return [
+    BrowserTarget(profile=profile, display_name=display_browser_name(profile, sock_path), socket_path=sock_path)
+    for profile, sock_path in active_endpoints(reg).items()
+  ]
+
+def is_active_local_profile(profile: str | None) -> bool:
+  """Return True when profile names a reachable local browser endpoint."""
+  if not profile:
+    return False
+  if REGISTRY_PATH.exists():
+    reg = load_registry(REGISTRY_PATH)
+    if profile in active_endpoints(reg):
+      return True
+  if not is_windows():
+    try:
+      return Path(endpoint_for_alias(profile)).exists()
+    except Exception:
+      return False
+  return False
+
+def resolve_socket(profile: str | None = None) -> str:
+  """Return the socket path for the given profile (or auto-detect)."""
+  target = profile or os.environ.get("BROWSER_CLI_PROFILE")
+
+  if target:
+    if REGISTRY_PATH.exists():
+      reg = load_registry(REGISTRY_PATH)
+      if target in reg:
+        return reg[target]
+    return endpoint_for_alias(target)
+
+  try:
+    active = active_local_browser_targets()
+    if len(active) > 1:
+      aliases = [target.profile for target in active]
+      examples = "\n".join(f"  browser-cli --browser {a} ..." for a in aliases)
+      raise BrowserNotConnected(
+        f"Multiple browser instances are active: {', '.join(aliases)}\n"
+        f"Use --browser <alias> to select one:\n{examples}"
+      )
+    if active:
+      return active[0].socket_path
+  except BrowserNotConnected:
+    raise
+  except Exception:
+    pass
+
+  raise BrowserNotConnected(
+    "Cannot resolve a browser socket automatically.\n"
+    "Make sure the browser is running with the browser-cli extension enabled,\n"
+    "or pass --browser <alias> / set BROWSER_CLI_PROFILE to a known alias."
+  )