Stop analysis tools from reading binary blobs

[derekwisong]

Stacked on #48.

Problem

Opening a large partitioned dataset and running an analysis tool (Describe, Distribution, or Correlation) made the program uninterruptible — q, Ctrl-C, and Ctrl-Z all did nothing, and the process had to be killed by PID. On a cloud-backed dataset this is also expensive (egresses every blob).

The freeze is not an event-loop bug — the loop stays responsive and allows quit while busy. The cause is that all three analysis tools collected the full `LazyFrame` including `Binary` columns:

• Describe — `build_describe_aggregation_exprs` aggregates every column
• Distribution — `compute_statistics_with_options` collects/samples the whole frame
• Correlation — `collect_lazy(lf)` over all columns

Those `Binary` columns hold multi-GB blobs, so the collect read every blob across every partition, exhausting memory until the process thrashed and could no longer service input. (Ctrl-C/Ctrl-Z are no-ops in raw mode regardless.)

`#48` stubbed binary columns in the display buffer but never the statistics path.

Fix

Route the analysis `LazyFrame` through the same binary-stub expressions the display buffer already uses (renamed `display_column_exprs` → `binary_stub_exprs`, now `pub(crate)`). Binary columns become a constant `‹binary›` string that is never read from disk:

• No blob reads → no OOM → no freeze, across all three analysis tools
• No cloud egress of blob bytes
• Binary columns still appear in describe, as a stub (real count, min/max = `‹binary›`) — no changes needed in `statistics.rs` or rendering

Scope note

This does not make a legitimately-expensive analysis (e.g. describe over a huge all-numeric dataset) cancellable — Polars `collect()` can't be aborted mid-flight today. That's deferred follow-up.

Test plan

• Added `analysis_describe_stubs_binary_columns_without_reading_blobs` — asserts describe yields the stub instead of reading the bytes
• `cargo test -p datui-lib` — 187 passed
• Manual: open the partitioned dataset with blob columns, run Describe — completes promptly, binary column shows `‹binary›`