Add type annotations and mypy CI to flatdata-py and flatdata-generator

[VeaaC]

Adds type annotations to all source functions/methods in both Python projects and
enforces them via mypy in CI.

What changed

Type annotations: All 415 functions across 62 source files are annotated.
This includes closure/nested functions (Jinja2 filters, data access readers,
validation helpers) and all properties.

mypy configuration (both pyproject.toml): 9 strictness flags enabled —
disallow_untyped_defs, check_untyped_defs, disallow_any_generics,
warn_return_any, warn_unused_configs, warn_redundant_casts,
warn_unused_ignores, no_implicit_optional, strict_equality. Namespace
package settings for the shared flatdata/ namespace.

CI: mypy steps added to generator.yml and py.yml, running after tests.

PEP 561: py.typed markers in flatdata/lib/ and flatdata/generator/.

Notable type improvements

• Node.iterate(), children_like(), first_parent_like() made generic via
  @overload/TypeVar — eliminates ~20 potential # type: ignore comments
  throughout the tree and builder code.
• Traversal namedtuples promoted to module-level typed NamedTuple classes
  (BfsAttr, DfsAttr).
• .doc properties typed as str (were Any). referenced_structures typed
  as list[BuiltinStructureReference | StructureReference] (was list[Any]).
• _raise correctly typed as NoReturn.
• Dead code removed (enumeration.py:_bits_required).

Remaining type: ignore (22 total)

All have specific error codes and justification comments. The majority (15) are
attr-defined on Reference.node — the node accessor returns Node but callers
access subclass attributes. Fixing this properly requires a Reference type
hierarchy refactor. The rest are standard mypy limitations (property narrowing,
ABC isinstance, set.add idiom, __repr__ override).

Not done

• strict = true evaluated, not adopted — adds ~100 errors from cross-package
  no-untyped-call and missing __all__ exports. Cost exceeds benefit.
• ReadableBuffer Protocol for data/mem parameters — blocked by Python 3.10
  support (collections.abc.Buffer requires 3.12).
• Storage/writer ABCs — structural refactor, out of scope.
• Test files remain unannotated (by design).