Add relation arrangement sidecar

[MesTTo]

Stack dependency: review after #70. This branch is based on the prerequisite head. Until the prerequisite PRs merge, GitHub may include their commits in the aggregate diff; the intended review increment is the new commit at this branch tip.

Adds a derived arrangement index for relation-like facts.

The branch introduces ArrangementDescriptor and ArrangementIndex over canonical term identities. It validates key orders, groups matching facts by selected argument positions, supports exact-key and prefix lookup, and compares suffix-bound arrangement results against the current product-query path.

Validation:

• cargo +nightly-2026-06-01 test -p mork --lib arrangements with RUSTFLAGS='-C target-cpu=native'
• cargo +nightly-2026-06-01 test -p mork --lib with RUSTFLAGS='-C target-cpu=native'
• rustfmt --check on kernel/src/arrangements.rs
• jscpd on kernel/src/arrangements.rs
• git diff --check
• normal diffstat matched git diff -w --stat

[MesTTo]

Closing: part of the un-wired binding_space sidecar line that reimplements the kernel's ProductZipper join (relational matcher / arrangements / trie-join / expression-trie) over a reinvented term_identity::TermId(u64), instead of reusing Expr and PathMap. The kernel core (space.rs/main.rs/sinks.rs/sources.rs/pure.rs) references none of these modules, so nothing is wired to the MeTTa surface. This is the 'reimplements rather than reuses / un-wired feature island' concern. Parking the line; if a specific piece is wanted (e.g. a new PathMap zipper type with property tests against existing ops), it can be reopened as a small standalone PR on PathMap primitives.