feat: add IterationSpeedColumn to progress

[PierreLapolla]

Type of changes

• Bug fix
• New feature
• Documentation / docstrings
• Tests
• Other

AI?

• AI was used to generate this PR

Generated with Claude Sonnet 4.6 via Claude Code, with human supervision and design decisions throughout.

Checklist

• I've run the latest black with default args on new code.
• I've updated CHANGELOG.md and CONTRIBUTORS.md where appropriate.
• I've added tests for new code.
• I accept that @willmcgugan may be pedantic in the code review.

Description

Adds IterationSpeedColumn, a progress column that displays iteration speed
(e.g. 1.2 it/s). For slow tasks (speed < 1 it/s), it displays inverted
time-per-iteration (e.g. 2.0 s/it), matching tqdm's behaviour. An optional
unit parameter (default "it") allows custom labels such as "sample/s" or
"epoch/s".

This feature was already approved and merged by @willmcgugan in #3332, then
reverted solely due to dynamic setattr on the Task dataclass
(task.last_speed = ...), which breaks static type checkers. This
implementation avoids that entirely — Task is never mutated. The column reads
task.finished_speed or task.speed, both existing fields already tracked by
the framework.

TaskProgressColumn.render_speed is also enhanced (backward compatibly) with
the same slow-task inversion and a style parameter, so unbounded tasks using
show_speed=True benefit too.

Related:

• Progress bars: printing iterations/second and unbounded processes #926 — original feature request
• Add new column object: IterationSpeedColumn #3332 — first implementation, merged then reverted
• Generic speed column for Progress #3533 — separate open attempt at the same feature