Preserve aggregation candidates during root merging

[snowyukitty]

Root aggregation merges now carry an explicit role: leaf fan-in remains bounded by segment_size, while root fan-in retains every leaf candidate until existing finalization performs the final prune. Focused coverage exercises leaf pruning, the leaf-to-root transition, root candidate preservation, and repeatable equivalent merges.

Tests, from the validation run on 40ba01f (2026-08-31, offline; not re-run since), from the quickwit/ workspace directory:

• cargo --offline test -p quickwit-search test_merge_intermediate_terms -- --nocapture — passes, covering the added test_merge_intermediate_terms_root_preserves_leaf_candidates
• cargo --offline check -p quickwit-search --tests
• rustfmt --edition 2024 --check quickwit-search/src/collector.rs quickwit-search/src/leaf.rs quickwit-search/src/root.rs
• the full offline quickwit-search suite: 207 of 207 passed

Whole-crate make fmt is not claimed.

Generated with OpenAI GPT-5.6 Sol via Codex.