Expose validated BindingSpace trie cursor contracts

[MesTTo]

Stack dependency: review after #90.

Adds an explicit cursor contract for replaying the selected BindingSpace trie traversal without materializing the join result.

The stack is split into three commits:

• expose per-factor cursor contexts through the selected-plan report
• validate exact ordered factor domains in trace summaries and replay diffs
• report monotone seek skips from lower-bound cursor movement

The formal LowerBound proof from the local development branch is intentionally excluded and will be proposed separately with the proof suite.

Validation:

• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib binding_space (16 passed)
• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib binding_plan (15 passed)
• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib
• jscpd --reporters ai --format rust kernel/src/binding_plan.rs kernel/src/binding_space.rs
• git diff --check

[MesTTo]

Closing with the rest of the binding_space line: the contract-and-validation shape is exactly what new zipper types should ship with, but here it validates the sidecar's bespoke cursor rather than a PathMap zipper. The rebuild is property tests for a real zipper type against existing PathMap ops, as a small standalone PR.