perf: make Rewrite transaction fragment handling O(n) instead of O(groups × fragments)

[tanish-exa]

Summary

handle_rewrite_fragments rebuilt each rewrite group by scanning the whole fragment vector, so a compaction commit cost O(groups × fragments). On a manifest with ~4.5M fragments and 128 groups per batch that dominates commit wall time. It now indexes once and rebuilds once:

let id_to_pos: HashMap<u64, usize> = ...;      // one pass over fragments
for group in groups {
    let positions = group.old_fragments.map(|f| id_to_pos[&f.id]);  // O(1) each
    removed[pos] = true;                        // mark, don't splice
    replacements.insert(min(positions), group.new_fragments);
}
fragments = rebuild(fragments, removed, replacements);  // one pass

Contiguous groups keep their replacements at the original position; non-contiguous groups append, preserving the previous ordering semantics.

Second commit adds the safety guard the position-indexed form needs: two groups replacing the same fragment id used to silently overwrite one replacement entry (the old scan-and-splice form couldn't express that state). It's now a CommitConflict, with a regression test.

Targets the exa/v9.0.1-txn-perf release line — the same change landed on main as #22 and was reverted by #23, since production wheels are cut from this branch. The 9.0.1+exa.6 version bump follows in its own PR, as with #20 → #21.

Link to Devin session: https://app.devin.ai/sessions/7d75a309bb144583a14b8704bbdb28e3
Requested by: @tanish-exa

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