Classify MeTTa special forms

[MesTTo]

Stack dependency: review after #92. The intended review increment is commit 957cabf.

Adds an exact-arity classifier for Minimal MeTTa and MeTTa special forms over the canonical term sidecar.

The classifier distinguishes supported machine forms from ordinary result atoms, unit, and error values. Arity checks prevent an expression with the same head symbol but a different shape from being treated as a special form.

Validation:

• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib formal_lowering
• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib
• rustfmt +nightly-2026-06-01 --edition 2024 --check kernel/src/formal_lowering.rs
• jscpd --reporters ai --format rust kernel/src/formal_lowering.rs
• git diff --check

[MesTTo]

Closing: the special-form classifier duplicates interpreter knowledge on a parallel term id; its parity-test value already lives in #109. Parking the classifier itself.