Check weighted sidecar arithmetic

[MesTTo]

Adds checked arithmetic to the weighted path sidecar.

set_weight, apply_delta, selection_tree, and select_by_offset_tree now report typed arithmetic errors instead of saturating or silently publishing an incorrect positive-weight total. The tests cover signed delta overflow, total positive-weight overflow, and no-mutation behavior on rejected updates.

Depends on #101.

Validation:

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

[MesTTo]

Closing as a duplicate of #101: this re-lands the whole weighted_paths.rs to switch saturating->checked arithmetic (~83% shared lines). Folding the checked arithmetic into #101 instead, so it lands as one PR checked-from-the-start.