Add DNF diagnostics and existence queries

[MesTTo]

Stack dependency: review after #95.

Adds diagnostic counters to the semantic PathMap DNF evaluator and an existence-only query that stops after the first non-empty clause instead of materializing the final union.

The tests cover factor reuse, clause/intermediate sizes, duplicate output accounting, and existence short-circuiting. A small test helper removes duplicated overlap-stat assertions without changing production behavior.

Validation:

• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib --features experimental_dnf path_dnf (5 passed)
• RUSTFLAGS='-C target-cpu=native -Awarnings' cargo +nightly-2026-06-01 test -p mork --lib --features experimental_dnf
• rustfmt +nightly-2026-06-01 --edition 2024 --check kernel/src/path_dnf.rs
• jscpd --reporters ai --format rust kernel/src/path_dnf.rs (0 clones)
• git diff --check

[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.