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/constants.py

@@ -0,0 +1,94 @@
+"""Project-wide constants for browser-cli.
+
+Only static values live here. Runtime-derived state (e.g. installed version,
+open sockets, locks) stays in the owning module.
+"""
+from __future__ import annotations
+
+import os
+from pathlib import Path
+
+APP_NAME = "browser-cli"
+RUNTIME_DIRNAME = ".browser_cli"
+DEFAULT_ALIAS = "default"
+
+NATIVE_HOST_NAME = "com.browsercli.host"
+EXTENSION_ID = "bfpmkhngkjnfhabmfckgeohlilokodkg"
+SUPPORTED_BROWSERS = ["chrome", "chromium", "brave", "edge", "vivaldi"]
+
+PROTOCOL_MIN_CLIENT = "0.9.0"
+MAX_MSG_BYTES = 32 * 1024 * 1024
+DEFAULT_REMOTE_PORT = 443
+DEFAULT_PAGE_SIZE = 100
+DEFAULT_TRANSPORT_THRESHOLD = 512
+
+NO_ROUTE_COMMANDS = {"browser-cli.targets", "browser-cli.auth.keys", "browser-cli.auth.trust"}
+GENTLE_MODES = ["auto", "normal", "gentle", "ultra"]
+
+PAGEABLE_COMMANDS = {
+  "tabs.list",
+  "tabs.filter",
+  "tabs.query",
+  "group.list",
+  "group.tabs",
+  "group.query",
+  "windows.list",
+  "dom.query",
+  "dom.text",
+  "dom.attr",
+  "extract.links",
+  "extract.images",
+  "extract.json",
+  "cookies.list",
+  "session.list",
+}
+
+NATIVE_HOST_DIRS = {
+  "chrome": {
+    "linux":  [Path.home() / ".config/google-chrome/NativeMessagingHosts"],
+    "darwin": [Path.home() / "Library/Application Support/Google/Chrome/NativeMessagingHosts"],
+  },
+  "chromium": {
+    "linux":  [Path.home() / ".config/chromium/NativeMessagingHosts"],
+    "darwin": [Path.home() / "Library/Application Support/Chromium/NativeMessagingHosts"],
+  },
+  "brave": {
+    "linux":  [Path.home() / ".config/BraveSoftware/Brave-Browser/NativeMessagingHosts"],
+    "darwin": [Path.home() / "Library/Application Support/BraveSoftware/Brave-Browser/NativeMessagingHosts"],
+  },
+  "edge": {
+    "linux":  [Path.home() / ".config/microsoft-edge/NativeMessagingHosts"],
+    "darwin": [Path.home() / "Library/Application Support/Microsoft Edge/NativeMessagingHosts"],
+  },
+  "vivaldi": {
+    "linux":  [Path.home() / ".config/vivaldi/NativeMessagingHosts"],
+    "darwin": [Path.home() / "Library/Application Support/Vivaldi/NativeMessagingHosts"],
+  },
+}
+
+WINDOWS_NATIVE_HOST_REGISTRY_KEYS = {
+  "chrome": [r"Software\Google\Chrome\NativeMessagingHosts"],
+  "chromium": [r"Software\Chromium\NativeMessagingHosts"],
+  "brave": [r"Software\BraveSoftware\Brave-Browser\NativeMessagingHosts"],
+  "edge": [r"Software\Microsoft\Edge\NativeMessagingHosts"],
+  "vivaldi": [r"Software\Vivaldi\NativeMessagingHosts"],
+}
+
+CONFIG_DIR = Path(os.environ.get("XDG_CONFIG_HOME", str(Path.home() / ".config"))) / APP_NAME
+DEFAULT_KEY_PATH = CONFIG_DIR / "client.key.pem"
+DEFAULT_AUTHORIZED_KEYS_PATH = CONFIG_DIR / "authorized_keys"
+
+SSH_AGENTC_REQUEST_IDENTITIES = 11
+SSH_AGENT_IDENTITIES_ANSWER = 12
+SSH_AGENTC_SIGN_REQUEST = 13
+SSH_AGENT_SIGN_RESPONSE = 14
+
+PQ_KEX_ALG = "ML-KEM-768"
+PQ_TRANSPORT_ALG = "ML-KEM-768+ChaCha20Poly1305"
+
+SER_JSON = 0
+SER_MSGPACK = 1
+COMP_NONE = 0
+COMP_ZLIB = 1
+COMP_GZIP = 2
+COMP_ZSTD = 3