#3212 [Other] serve: support IPv6 bind addresses

Beschreibung

## Problem and result
Starting Strata with an IPv6 bind address such as `--host ::` failed before model loading because the HTTP server always used an IPv4 socket. The server now selects `AF_INET6` for IPv6 literals, binds `::`, serves requests through `::1`, and formats IPv6 URLs with brackets. IPv4 behavior remains unchanged.

## Related discussion
Supersedes closed PR #1326: https://github.com/Niko1221/Strata/pull/1326

## Approach
Add an IPv6 `Server` subclass and select it for IPv6 literals in both the startup preflight and the running server. Advertise the loopback URL as `[::1]` for `--host ::`, and document the IPv6 option. No default or authentication behavior changes; an API key remains recommended when listening beyond loopback.

## Validation

**Passed:**
- `python3 -m py_compile serve/server.py serve/test_security.py setup.py` at `fb2f6952` on macOS.
- `PYTHONPATH=<temporary requirements target> python3 -m unittest serve.test_security -v`: 28 tests passed, including a real `::` bind and `/health` request through `::1`.
- `git diff --check origin/main...HEAD`: passed.

**Failed:**
- `PYTHONPATH=<temporary requirements target> python3 -m unittest serve.test_server -q`: 341 tests ran; one existing vision test failed because `serve/lifeline.py` expects a mocked process to expose `.pid` under Python 3.14. This is unrelated to the IPv6 changes.

**Not run:**
- CUDA, HIP, SYCL and Metal builds; this change only touches the Python server, setup help, tests and documentation.

## Limits
IPv6 support depends on the host OS having IPv6 enabled. Listening beyond loopback still requires an API key in normal startup.