Prune empty DNF clauses

[MesTTo]

Prunes DNF clause evaluation after an intermediate conjunction becomes empty.

This keeps the feature-gated semantic evaluator from inspecting later factors that cannot affect a clause result. It also reports skipped factor positions so the tests can distinguish pruned work from evaluated work.

Depends on #96.

Validation:

• CARGO_HOME=/home/user/Dev/.cargo-isolated RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly test -p mork --features experimental_dnf path_dnf --lib
• CARGO_HOME=/home/user/Dev/.cargo-isolated RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly test -p mork --features experimental_dnf --lib
• rustfmt --check kernel/src/path_dnf.rs
• jscpd --reporters ai --min-lines 5 --min-tokens 50 kernel/src/path_dnf.rs

[MesTTo]

Folded into #95: the DNF line was one unit split as serialized rewrites of the same function bodies (each PR restructured the previous one's code), so it is now a single coherent feature-gated PR — the reference evaluator with instrumentation, differentially sealed against PathMap's fused zipper_merge_dnf, with the empty-clause convention divergence pinned. See #95.