fix(parquet): coalesce push decoder retained buffers

[cakeni]

Which issue does this PR close?

Closes #9695.

Summary

• replace quadratic exact-range clearing with indexed push-buffer ownership
• split coalesced pushed ranges into independently releasable row-group buffers
• release staged bytes after constructing each row-group reader
• preserve forward, reverse, duplicate, and incremental row-group decoding

Validation

• cargo test -p parquet push_buffers
• cargo test -p parquet push_decoder
• cargo fmt --all -- --check
• cargo clippy -p parquet --all-targets --all-features -- -D warnings