Add FormalMeTTa exec lowering sidecar

[MesTTo]

Stack dependency: review after #69. This branch uses the term-identity pattern API for read-only fact iteration. The intended review increment is commit 496e5fc.

Adds a derived FormalMeTTa planning view for encoded exec terms without changing the authoritative atom store or runtime semantics.

The lowering classifies source-list modes, path/equality/inequality sources, add/remove effects, later-step exec outputs, and variable context holes. The result is an inspection and planning API for later execution work.

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: this exec-lowering sidecar duplicates the interpreter's exec handling over a parallel term identity and is un-wired / planner-flavored. Parking it.