feat(usd): add UndoStage::discard_oldest

[bresilla]

Another small one from the same batch as #139.

with_capacity drops the oldest transaction once the stack is full, but in an editor one undo step is usually a few transactions (even define_prim + set_type_name is two). So eviction can cut a command in half, and undoing it later only reverts part of it.

This adds discard_oldest(count) so the stage can stay unbounded and the editor trims the history itself, on its own command boundaries. It's basically a partial reset: it flushes pending edits first, drops up to count of the oldest transactions, leaves the scene alone and returns how many it dropped.

C++ doesn't have an undo stack at all (it just reports undoable edits to SdfLayerStateDelegate and leaves history to the app), so this is only on the Rust side.

[Copilot]

Copilot review overview

🟢 Approval recommended

The focused implementation has behavioral coverage, and only minor test-naming feedback remains.

Review effort: Balanced
Findings: 2

Open (2)

• Test name exceeds naming convention · New
• Test name exceeds naming convention · New

What changed in this PR

Adds UndoStage::discard_oldest(count) so editors can trim undo history at
command boundaries while preserving the current scene.

Changes:

• Flushes pending edits, removes up to count oldest transactions, and returns the number removed.
• Tests retained undo behavior, scene preservation, count limits, and pending direct edits.

File	Description
crates/​openusd/​src/​usd/​capture.rs	Adds the history-trimming method, documentation, and tests.

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[mxpv]

I think the underlying issue is that one logical undo step can span several transactions. discard_oldest(count) leaves the editor tracking those boundaries through transaction counts, which feels fragile.

What do you think of an API like this:

undo.group(|stage| {
    stage.define_prim("/World")?.set_type_name("Xform")?;
    Ok(())
})?;

undo.undo()?; // Undoes both transactions.

Ordinary actions need no wrapper.
Use group only when several actions should undo together.

This lets the editor define undo boundaries directly, preserves immediate visibility of committed edits, and ensures undo and history eviction always operate on complete user actions.