compute, timely-util: tech debt cleanup

[antiguru]

Motivation

Bundle of mechanical cleanups identified during a tech-debt audit of src/compute and src/timely-util (CLU-81). One PR to avoid spam; each item is independent and can be reviewed in isolation by file.

Description

• timely-util/columnation: add SAFETY comments on every unsafe block in ColumnationStack (copy, clear, two copy_destructured). Project rule: every unsafe needs a SAFETY justification. Invariant: Region::copy returns aliasing items; local: Vec<T> is a tombstone whose Drop glue is suppressed via set_len(0) before drop.

• timely-util/reclock: document the pusher contract under a # Pusher contract heading. The function returns a Box<dyn Push<...>> that callers must drive — if it is dropped without ever being used, the operator silently deadlocks at the consumer.

• timely-util/scope_label: re-implement the timely-scope profiling label that was gutted in the timely v4 upgrade (was a no-op with_label(self) -> Self). New design wraps the body of a dataflow in a same-timestamp subgraph installed as a LabelledOperator; each schedule() call runs inside a custom_labels scope tagged timely-scope=<name>. The pre-v4 in-place Scope wrapper is no longer expressible (timely v4 made Scope a concrete struct), so the API changes from let scope = scope.with_label(); to scope.with_label(|scope| { ... }). All seven call sites in compute and storage are migrated. There is one extra subgraph layer per dataflow now, with associated progress-tracking overhead.

• compute/arrangement: add debug_assert! for the PaddedTrace invariant (padded_since strictly less than the inner trace's logical compaction frontier) at both mutation sites in into_padded and set_logical_compaction. The invariant was previously comment-only and unenforced.

The large line counts in the storage/compute render.rs files are mostly cargo fmt re-indentation from wrapping the dataflow body in a closure; the actual semantic change at each site is one extra level of nesting.

Verification

• cargo check --workspace passes.
• cargo clippy --workspace --all-targets -- -D warnings passes.
• bin/lint rustfmt step passes (other unrelated lint failures: missing buf, trufflehog, protobuf tooling in this environment).