Promote columnar Column/ColumnBuilder into the container crate

[frankmcsherry]

Moves the columnar container out of examples/columnar.rs and into timely_container, so it becomes a first-class, documented container rather than something users copy out of the example.

What changes

• Column<C> is now a type alias for columnar's Stash<C, Bytes>, with the container traits (Accountable, DrainContainer, SizableContainer, PushInto) implemented directly on it. No wrapper struct, and no re-statement of Stash's typed/bytes/aligned representation — the previous example newtype only delegated to Stash.
• container gains a columnar (+ timely_bytes) dependency. It already carries concrete container impls (Vec<T>, …), so this isn't a new kind of dependency; it teaches the container layer about columnar, which lower crates (e.g. communication, already a columnar dependent) can also use.
• The wire encoding stays in timely. dataflow::channels::ContainerBytes is a timely trait, so its impl for the alias lives there (a local trait on a foreign type — no orphan issue), next to the existing Vec<T> impl.
• ColumnBuilder moves across unchanged, now with a comment recording why it encodes eagerly: it keeps the (expensive-to-rebuild) typed accumulator resident, so an exchanged batch — sent to a peer and never returned — need not round-trip to be recycled. Behavior is identical.
• examples/columnar.rs shrinks to use timely::container::{Column, ColumnBuilder} over the same dataflow (~125 fewer lines), so it now exercises the public API.

Convenience alias ColumnOf<T> and a named COLUMN_CAPACITY_BYTES constant (previously a magic 1 << 20) are included.

Notes

• No default is changed: Vec<T> + bincode remains the default container. This is purely additive.
• The at_capacity threshold (1 MiB) is preserved as a documented default; tuning it is deliberately left as a bring-your-own-builder concern.

Verified locally: cargo build --workspace --all-targets, clippy clean on the new module, and examples/columnar runs and produces the expected wordcount output.

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