Add ACTOutputStream: build an ACT on disk from a stream of ordered paths

[imlvts]

Streams strictly-increasing paths into an on-disk ArenaCompactTree without holding the trie in memory: a stack of in-construction frames (one per byte of the current path) is sealed bottom-up whenever the input diverges, writing each node's children contiguously as the format requires and folding non-branching runs into line nodes. Reuses the existing FileDumper writer, including line-data dedup.

    let mut b = ACTOutputStream::new("file.act")?;
    b.push("123")?;
    b.push_val("124", 42)?;
    let tree = b.finish()?;  // mmap-backed ArenaCompactTree

Also fixes a pre-existing out-of-bounds walk in get_val_at: it never checked the branch bytemask before index_of/nth_node, so looking up a byte absent from a branch with children stepped past the last sibling into unrelated arena bytes (subtract-overflow panic in debug builds).