config: retire ServerArgs.override in favour of derive()

`ServerArgs.override(source, **fields)` was the last way to change a resolved
`ServerArgs` in place. Every remaining call-site was one of two things, and
neither wanted an in-place write:

- **A config for someone else.** A draft worker's context length, an encode
  worker's device, the compile script's watchdog, the client's port pick, a test
  fixture's backends. These already deepcopied first — the write was on the copy.
- **A launcher-stage resolution.** `resolve_auto_parsers` detected the chat
  template's parsers and wrote them back, to be inherited by the schedulers it
  spawns.

Both are "one config becomes another", so `derive(source, **fields)` returns the
variant and leaves the receiver — and any bags projected from it — untouched. It
deliberately is not `dataclasses.replace`: resolution does not re-run, because
the values being set are decided after it, from inputs it never had. Provenance
and the resolvable-field stash work as before, on the copy.

`resolve_auto_parsers` now computes the parsers and returns the config to launch
with; the detection helpers stop taking a config to mutate. `HiMambaRadixCache`
re-applied a HiCache layout normalization `__post_init__` already performs (the
same duplicate removed from `UnifiedRadixCache` in ebb1c88d23) and just goes.

With no in-place mutation left, `ServerArgs.__setattr__` raising after
resolution *is* the guarantee, so the textual writer ratchet retires and
`test_server_args_derive.py` pins the contract instead: the receiver survives
deriving, the published instance still refuses assignment, and deriving does not
publish. `SGLANG_STRICT_CONFIG_MUTATION` was already unused — the guard has been
unconditional since the mutation sweep — and goes with it.

The detection tests drop their `SimpleNamespace` stand-in for a real
`ServerArgs`; the test kit and the MLA chunk-metadata fixture publish a derived
variant instead of writing the runner's published config.

test/registered/unit/test_runtime_context.py

@@ -299,12 +299,13 @@ class TestServerArgsScopedOverride(_IsolatedServerArgs):
 
     def test_installed_config_arms_the_strict_guard(self):
         # The published dummy must behave like a resolved config: bare writes
-        # raise under the strict harness; override() stays the entry point.
+        # raise, and a differing value lives on a derived variant.
         published = get_context().override_server_args(tp_size=2).install()
         with self.assertRaises(AttributeError):
             published.tp_size = 4
-        published.override(source="test", tp_size=4)
-        self.assertEqual(published.tp_size, 4)
+        variant = published.derive("test", tp_size=4)
+        self.assertEqual(variant.tp_size, 4)
+        self.assertEqual(published.tp_size, 2)
 
     def test_restore_resets_the_capture_seed(self):
         # install() seeds flags.capture from the published dummy; restore()