feat(query): add typed bounded-reachability capability

[ds1sqe]

Summary

The typed query API can express fixed-hop relation chains, but not a
path with an explicit minimum and maximum depth. Selecting every
entity reachable through a recursive relation (e.g. all descendants
below one root through Composition(parent, child)) within a
caller-declared depth bound currently forces consumers into
client-side fallbacks: one request per hop, client-side joins, or
looping until no new nodes appear.

Proposed change

Add a bounded-path expression to the typed query API: bind a start
variable, an end variable, a relation type, ordered endpoint roles,
min_depth, and a finite max_depth, without degrading the inferred
Query[T] result type.

Acceptance criteria

• Path identity, cycle handling, zero-length behavior,
  duplicate-path versus selected-row distinctness, subtype
  matching, and stable ordering are specified and covered by
  tests.
• Invalid bounds fail at query construction, before any
  database I/O.
• A test fixture schema with nested membership (depth >= 2,
  shared subtrees, and at least one cycle) verifies the distinct
  selected result set at each depth bound.
• The Python bindings infer the exact typed result without Any,
  casts, or ignore directives.

Related

• Part of the typed-Query epic (epic: add one typed Query facade over Rust-owned match plans #170-feat: add identity-safe page_by execution #177).