Speed up Application/Configurable instance startup

[Carreau]

Speeds up the runtime cost of building an Application and its many
Configurable sub-components at startup — the pattern used by IPython, Jupyter
server, ipywidgets, etc., which define hundreds of HasTraits subclasses at
import and instantiate a large component graph at launch.

The import-time half of this work ("Defer heavy imports to speed up import time")
has already landed separately on main (626bbe5), so this PR is now purely the
runtime optimizations. Each commit is independent and can be reviewed (or pulled)
on its own.

Commits

1. Reuse the class-namespace walk across the two metaclasses.
   MetaHasDescriptors.setup_class already walks the full class namespace via
   getmembers(cls); it now returns that (name, value) list so
   MetaHasTraits.setup_class can reuse it to find TraitType members instead
   of doing a second, redundant dir(cls) + getattr walk over every class.
   Same members in the same order, so semantics are identical (getmembers
   already skips members whose getattr raises, which is what the removed
   try/except handled).
   → class definition ~118 µs → ~96 µs per class.

2. Cache metadata-filtered class_traits()/traits() results per class.
   Filtering by metadata (e.g. class_traits(config=True)) was recomputed from
   scratch on every call — the single largest cost of Application startup
   (~25-30%), invoked ~45× per startup for results that are static per class
   (from Application._classes_with_config_traits,
   KVArgParseConfigLoader._add_arguments, and each Configurable._load_config).
   Both methods now delegate to a shared classmethod that memoizes the filtered
   dict per class and returns a .copy(), preserving the existing "fresh dict"
   contract (the cached dict never escapes by reference). cls._traits is frozen
   after class creation (add_traits() builds a new class rather than mutating),
   so the only way a filtered result can change is a post-hoc metadata mutation
   via tag()/set_metadata(); those bump a module-level generation counter and
   stale cache entries are recomputed. Only constant (non-callable, hashable)
   filters are cached; callable predicates stay on the uncached path.
   → class_traits(config=True) ~8.3 µs → ~1.3 µs per call.

   Reviewer note: the cache is invalidated by the supported post-construction
   metadata APIs (tag()/set_metadata()). Mutating trait.metadata as a raw
   dict after the class has already been queried is not reflected until the next
   generation bump — a pattern not used in traitlets and vanishingly rare in
   practice, but called out for awareness.

3. Skip redundant re-validation of constructor kwargs.
   HasTraits.__init__ validated every trait kwarg twice: once via setattr in
   the fast loop, then again via _cross_validate + set_trait. The second pass
   is only needed for traits that actually have a cross-validator; the loop now
   guards on the same condition _cross_validate itself uses and, for traits
   without one, records the already-stored (possibly coerced) value for the
   notification instead of re-validating. Notification payloads are byte-for-byte
   identical.
   → instantiation with kwargs and no cross-validators ~1.2-1.4× faster.

4. Skip config loading for Configurables with no matching config.
   Configurable._load_config computed traits(config=True) and entered
   hold_trait_notifications() unconditionally, even for the many leaf
   Configurables whose config has no matching keys. It now computes my_config
   first and returns early when empty. Also removes a dead
   section_names = self.section_names() local that was computed (twice per
   instance) but never used.
   → ~1.2× on leaf Configurables with no matching config.

End-to-end

On a synthetic app modeled on Jupyter/IPython scale (12 Configurable
components, 373 traits, config applied), Application() construct + initialize()
drops ~2196 µs → ~1740 µs (~21%) on Python 3.11. The wins compound with the
number of Configurables built and traits scanned at startup.

All changes are behavior-preserving. Full test suite passes (including new tests
covering the cache's invalidation/copy contract, unhashable/callable filters, and
the deferred-import cold paths), along with mypy and ruff.

[Carreau]

It does not affects ipython startup visibly, but does hove an effect on raw 'python -c "import traitlets.config"'

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