Reduce constant projections through select and extension storage - #11056

Summary

Extracting a constant extension-valued field through select, then extracting its storage, can miss the immediate-input constant execution paths and request canonical arrays. Reduce these operations during expression and array optimization so the result remains constant.

For example, the regression test uses an input struct with a constant measurement field whose dtype is an extension over storage {value: i32}, alongside a varying other field:

get_item(
    "value",
    ext_storage(get_item("measurement", select(["measurement"], root()))),
)

Here the inner get_item returns the extension-valued measurement field. ext_storage unwraps that value to its storage struct, and the outer get_item extracts value.

Changes

• Bypass include/exclude selections when extracting a retained field.
• Reduce get_item on constant structs and ext_storage on constant extensions, sharing the existing field extraction logic with execution.
• Add regression coverage for the nested chain, struct validity, null constants, empty arrays, and removed fields.

The projection rewrite currently uses the shared scalar-function reducer. Moving array rewrites into the session kernel registry requires wiring session-aware optimization into the relevant callers and is left for follow-up.

Validation

• Passed cargo +nightly-2026-09-10 fmt -p vortex-array.
• Passed cargo clippy -p vortex-array --all-targets --all-features -- -D warnings.
• Tests were not run locally.