Phase 1, Task 1: Fix encoder finalization in process_through_compression

[aram356]

Parent: #563
Phase: #566

Summary

process_through_compression uses flush() then drop(encoder) which silently swallows finalization errors. This is a prerequisite for Task 2 (moving gzip to this path) — without it, gzip would regress from working finish() to broken drop().

Changes

• Change signature from W (by value) to &mut W so caller retains ownership for finish()
• Remove drop(encoder) — caller calls finish() explicitly
• Update process_deflate_to_deflate and process_brotli_to_brotli callers
• Add deflate round-trip correctness test

File: crates/trusted-server-core/src/streaming_processor.rs:334-393

Plan

See docs/superpowers/plans/2026-03-25-streaming-response.md — Task 1