Avoid expanding unreferenced struct plan fields - #9254

Summary

• keep direct bound GetItem(root) expressions intact when partitioning a struct plan
• expand Select(root) into only the selected fields
• expand every field only when an expression genuinely consumes the bare struct root
• preserve include/exclude projection semantics with focused plan snapshots

Root cause

ExpressionStructRule eagerly built a bound pack containing every struct field before it knew
which fields the query referenced. On the 105-field ClickBench schema, every rule application
constructed 105 GetItem expressions. Binding each node resolved its return dtype, repeatedly
deserializing lazy field dtypes even for a one-column query.

The expression is already bound, and the bound field annotator can partition direct field access
without expanding it. This change only synthesizes field accesses where Select or a bare root
semantically requires them.

Performance

The benchmark opens all 100 ClickBench Vortex shards, filters AdvEngineID != 0, projects
AdvEngineID, warms planning once, and reports the median of 25 complete planning passes.

This is a 66.6% planning reduction from the bound baseline and a 70.3% reduction from the original
stack. Execution is unchanged within run-to-run noise.

A matched Samply planning-only profile over 500 all-file passes reduced main-thread CPU from
8.22 s to 1.23 s. The previous hot stack through BoundExpression::try_new,
GetItem::return_dtype, and lazy dtype deserialization is absent after the change.

Stack

This draft is stacked on #9244 and contains one signed commit.

Validation

• RUSTC_WRAPPER= cargo nextest run -p vortex-layout -p vortex-scan-v2 — 215 passed
• RUSTC_WRAPPER= cargo clippy --all-targets --all-features -- -D warnings
• RUSTC_WRAPPER= cargo test --doc -p vortex-layout -p vortex-scan-v2
• cargo +nightly fmt --all -- --check
• git diff --check