table: Add ranged dumps for batched export (#2755)

Closes #2754

## Description

`TableState::dump_range` lets callers materialize a selected row range
while
preserving the existing full-table `dump` API. The requested range is
clamped
to the delegate's current row count so applications can iterate over
large
tables in fixed-size batches.

The DataTable story now prompts for the destination before reading table
data,
then materializes 2,000 rows at a time on the foreground GPUI context. A
capacity-one channel sends those batches to one background
`csv::Writer<File>`,
which streams directly to disk without accumulating the complete table,
CSV
buffer, or `String` in memory.

`TableState::dump` remains an eager whole-table API for compatibility.
Large
data exporters should use `dump_range` with an application-level batch
size.
Progress UI, cancellation, remote pagination, and delegate changes
remain
outside this PR.

## Screenshot

Not applicable. This changes export scheduling and file I/O rather than
the
rendered table UI.

## How to Test

```sh
cargo fmt --all -- --check
cargo test -p gpui-component --test table_dump_range
cargo test -p gpui-component
cargo check -p gpui-component-story
RUSTC_BOOTSTRAP=1 cargo check \
  -p gpui-component-story-web \
  --target wasm32-unknown-unknown
```

Observed results:

- formatting passed;
- both `table_dump_range` tests passed;
- the complete `gpui-component` test suite passed;
- the native story crate compiled successfully;
- the WASM story check passed after enabling the unstable feature
required by
the current `wasm_thread` dependency (`RUSTC_BOOTSTRAP=1`); a plain
stable
check stops in that dependency with `E0554` before compiling the story;
- the DataTable story opened its save dialog before export work;
cancelling
produced no output, while exporting the default table produced one
header
  and 5,000 records with 45 columns each;
- the first exported record had ID `0`, the last had ID `4999`, and the
CSV
  parsed successfully with consistent column counts.

The ranged dump regression tests verify that `dump_range(98..103)` is
clamped
to a 100-row table and only calls `cell_text` for rows 98 and 99, while
the
existing `dump(cx)` still returns the complete table.

## AI Assistance

AI assisted with root-cause analysis, implementation, test design,
validation
planning, and PR wording. I reviewed the final diff and ran the checks
listed
above.

## Checklist

- [x] I have read the [CONTRIBUTING](../CONTRIBUTING.md) document and
followed the guidelines.
- [x] Reviewed the changes in this PR and confirmed AI-generated code is
accurate.
- [x] Passed `cargo run` for story tests related to the changes.
- [ ] Tested macOS, Windows, and Linux platform performance (not
platform-specific).

---------

Co-authored-by: Floyd Wang <gassnake999@gmail.com>