refactor(api): namespaced SDK + dedicated transport layer

Restructure the Python API and internals around composable namespaces and
a standalone transport/endpoint layer. Bump to 0.12.0.

Python API:
- Replace flat methods (b.tabs_list(), b.group_list()) with namespaces:
  b.nav, b.tabs, b.groups, b.windows, b.dom, b.extract, b.page, b.storage,
  b.cookies, b.session, b.perf, b.extension.
- Shrink browser_cli/__init__.py to a thin composition root; move all
  behaviour into browser_cli/sdk/ (one module per namespace + factories,
  base, routing).

Internals:
- Add browser_cli/transport.py and remote_transport.py to isolate IPC from
  command logic; client.py now delegates instead of owning transport.
- Add browser_cli/endpoints.py for endpoint resolution and
  browser_cli/errors.py for shared error types.
- Extract markdown rendering into browser_cli/markdown.py (out of extract).
- Add USER_AGENT to version_manager.

Tooling & tests:
- Add justfile with common dev tasks.
- Update CLI commands and demo to the namespaced API.
- Rework tests for the new layout; add test_transport.py and
  test_refactor_boundaries.py to lock in module boundaries.

BREAKING CHANGE: flat API methods are removed in favour of namespaces
(e.g. b.tabs_list() -> b.tabs.list(), b.group_list() -> b.groups.list()).

pyproject.toml

@@ -1,14 +1,19 @@
 [project]
 name = "browser-cli"
-version = "0.10.4"
+version = "0.12.0"
 description = "Control your real running browser from the terminal or Python SDK"
 requires-python = ">=3.10"
 dependencies = [
     "click>=8",
     "cryptography>=48",
     "rich>=13",
+    "msgpack>=1",
 ]
 
+[project.optional-dependencies]
+# Better/faster remote response compression than the stdlib zlib/gzip fallback.
+fast = ["zstandard>=0.22"]
+
 [project.scripts]
 browser-cli = "browser_cli.cli:main"
 
@@ -16,6 +21,7 @@ browser-cli = "browser_cli.cli:main"
 dev = [
     "pytest>=8",
     "pytest-cov>=7.1.0",
+    "zstandard>=0.22",
 ]
 
 [build-system]