perf: defer optional typing imports (#634)

## Summary

- resolve heavy optional typing exports lazily on first access
- preserve precise analyzer-visible vendor types when their extras are
installed
- preserve stable runtime shims and Protocol fallbacks when optional
dependencies are absent
- defer optional imports in serializers, storage, schema, and type-guard
consumers
- keep `DEFAULT_TYPE_ENCODERS` fully dict-compatible with thread-safe,
failure-retryable lazy registration

## Compatibility

Users do not need optional libraries installed for SQLSpec imports or
runtime annotations to work. Missing dependencies resolve to stable
cached shims; installed dependencies retain their precise static vendor
types. In a no-extras consumer type-checking environment, optional
vendor aliases resolve as `Any` because the vendor type definitions are
unavailable; the explicit SQLSpec Stub and Protocol types remain
available where callers need a dependency-independent contract.

Public exports, `dir()`, identity, pickle/repr behavior, cattrs fallback
names, dict mutation/union behavior, and runtime type-adapter behavior
are covered.

## Performance

Bare `import sqlspec` median time decreased from 681 ms to 202.467 ms
(70.27%). The import regression guard confirms pandas, polars, PyArrow,
Litestar, Pydantic, OpenTelemetry, and Prometheus are not imported
eagerly.

## Validation

- lazy-export, fallback-shim, optional-boundary, serializer, storage,
concurrency, retry, and dict-contract tests
- Ruff, mypy, pyright, docs, compiled acceptance tests, and MyPyC wheel
build
- full GitHub unit/integration matrix on Python 3.10-3.14

All required GitHub checks pass on the final commit.

Author: cofin

docs/reference/typing.rst

@@ -7,6 +7,29 @@ SQLSpec.
 
 .. currentmodule:: sqlspec.typing
 
+Optional dependency exports
+===========================
+
+The :mod:`sqlspec.typing` and :mod:`sqlspec._typing` modules resolve heavy
+optional-dependency symbols lazily. Importing :mod:`sqlspec` does not import
+Pydantic, Litestar, PyArrow, pandas, Polars, OpenTelemetry, or Prometheus.
+Accessing one of their exported symbols imports its dependency on first use and
+caches the resolved object::
+
+   import sqlspec.typing as sqlspec_typing
+
+   arrow_table_type = sqlspec_typing.ArrowTable  # Imports PyArrow here.
+
+When an optional dependency is not installed, the same access returns a stable
+typed shim. Repeated access returns the identical real object or shim, preserving
+annotation and runtime identity behavior. Features that require the dependency
+still raise the normal missing-dependency error when enabled.
+
+``msgspec`` remains eager because its core types and conversion functions are
+used throughout result mapping and serialization. ``orjson`` also remains on
+the existing eager path; both have a small measured import cost compared with
+the deferred integrations.
+
 Metadata Types
 ==============