Add fixed binding environment

[MesTTo]

Adds a fixed 64-slot binding environment on top of the term identity sidecar.

The branch provides BindingEnv, Binding, snapshots, marks, scoped rollback, and slot iteration for MORK's six-bit variable domain. Slots are tracked with a bitmask and rollback trail so repeated variables can be checked without heap maps. The environment stores TermId and optional FactId, so this branch is stacked on the term identity sidecar branch.

Validation:

• cargo +nightly-2026-06-01 test -p mork --test binding_env 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/binding_env.rs and kernel/tests/binding_env.rs
• jscpd on kernel/src/binding_env.rs and kernel/tests/binding_env.rs
• git diff --check
• normal diffstat matched git diff -w --stat, so the branch is not inflated by whitespace formatting