feat(db-python): read-side bindings with type registry and annotations

[EnriqueParodi]

pybind11 bindings for sen::db that let a Python script open a recording,
walk a cursor, and reflect on every payload through the same type system
the recorder used.

Surface:

• sen.Input(path): opens a recording. Owns its CustomTypeRegistry; a
  process can open several recordings independently.
• Input.types: live view of the registry (Python class objects, dynamic
  via getattr so unknown names raise AttributeError fast).
• Cursor: forward iterator over recorded events; advance(), payload,
  timestamp; entry.payload is a tagged union dispatched by isinstance.
• Snapshot / Creation: object views with reflection. objectId surfaces
  the recorder's stable id (renamed from the ambiguous .id).
• KeyedVar: dict-shaped (tag -> value); reviewed and kept as dict for
  ergonomic reasons (the class-shape alternative was rejected).
• Annotation surfacing on every entry; annotation.type goes through
  asCustomType so the same reflection path covers it.

Correctness commitments:

• ObjectId pybind11 caster validates range + signedness + error path
  (previously could silently truncate / accept negatives).
• typeNeedsAdaptation cache keys by Type* not MemberHash (MemberHash
  collisions between unrelated types in different recordings were
  reachable).
• Payload copies into Python so keep_alive lifetime chains hold across
  cursor advance.
• Annotation.type uses asCustomType; docstring matches behaviour.
• InputPy clears its lookupProperty cache on destruction (no stale
  pointers if a second Input opens the same recording shape).
• Built-in dispatch is O(1) via lookup table, not the prior linear scan.
• propertyNames cached per ClassType (not recomputed per Snapshot).
• pybind11 idiom + include hygiene: const-ref string visitors,
  TypeRegistryPy cached on the module, std::filesystem in headers.

Testing:

• bindings smoke test: opens a small recording fixture, walks one
  cursor, asserts on the reflected types. Catches "file deleted but
  module still compiles" cases that pure C++ build verification misses.
• CMake piggybacks on SEN_BUILD_PY (no separate option fan-out per
  per-element-with rule).

Documentation:

• users-guide page + db_library cross-ref + Tier 2 walk example
  (a small Python script that opens a recording and aggregates
  per-class state, runs as written).