VarBinViewBuilder: support decoding Dict and Sparse arrays directly into the builder - #9236

Dict was the one string encoding with no direct VarBinViewBuilder path:
it took the dictionary to full logical length (allocating an
intermediate views buffer proportional to the row count) and then
append_varbinview_array walked all those views a second time to rebase
their buffer indices. Sparse had no append_to_builder override at all
and paid the same double pass over its scattered views.

- VarBinViewBuilder grows push_buffers (flush-aware, dedup-aware buffer
  adoption returning the index each buffer landed at) and two bulk view
  appends built on it: append_views_gathered (one rebased view write per
  row through an index lookup, null rows skip the lookup) and
  append_views_scattered (one bulk fill-view write plus one write per
  patch).
- Dict gathers views through its codes straight into the builder: the
  dictionary's buffers are adopted once — deduplicated across chunks
  sharing the dictionary — with no byte copy and no intermediate array.
- Sparse overrides append_to_builder for strings: the view builder gets
  the scatter directly, and VarBinBuilder gets an in-order walk of fill
  runs (append_n_values) and patches, preserving last-wins semantics for
  duplicate patch indices. Non-string dtypes keep the canonicalize
  fallback.
- execute_varbin_inner no longer pushes a data buffer for a fill value
  short enough to inline — the view never referenced it, so it was pure
  dead weight in every canonical decode of a short-filled sparse array.

Checks: cargo nextest -p vortex-array -p vortex-sparse; cargo +nightly
fmt --all; cargo clippy --all-targets on the touched crates.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01Uet46qdrbQcF61HXRG7EKc
Signed-off-by: Robert Kruszewski <github@robertk.io>