refactor: defer morsel decoder build to Morsel::into_stream

The previous `build_stream` built every morsel's `RowFilter`,
`ParquetPushDecoder`, `AsyncFileReader`, and `Projector` eagerly in a
single loop inside the file planner — before any morsel was scheduled.
That loop ran on the scheduler thread and was visible as a 10–15%
regression vs. main on ClickBench-partitioned queries that have many
row-group morsels per file (e.g. Q15, Q16 at pushdown=off).

Replace `ParquetStreamMorsel` (which held a pre-built `BoxStream`) with
`ParquetLazyMorsel`, which holds only the per-chunk `ParquetAccessPlan`
plus an `Arc<LazyMorselShared>` of the file-level state. The decoder
and reader are constructed inside `Morsel::into_stream`, so each
morsel pays its setup cost only when the scheduler actually picks it
up, and the work is distributed across worker threads instead of
serialised on the planner.

`FilePruner` is `!Clone` and drives whole-file early-stop via
`EarlyStoppingStream`, so it still lives on chunk 0's morsel only.
The warm `async_file_reader` from metadata / page-index / bloom-filter
load is dropped at the end of `build_stream` — every morsel mints a
fresh reader via the factory at `into_stream` time. For both built-in
factories (`DefaultParquetFileReaderFactory`,
`CachedParquetFileReaderFactory`) the "warm cache" benefit of reusing
a reader is negligible because the underlying `Arc<dyn ObjectStore>` /
`Arc<dyn FileMetadataCache>` is already shared across readers, so the
simplification is free.

Local ClickBench-partitioned, 10 iterations, pushdown=off (M-series):

| Query | main  | eager (before) | lazy (this commit) |
|-------|------:|---------------:|-------------------:|
| Q14   |  325  | 335            |             313 ms |
| Q15   |  309  | 358            |             302 ms |
| Q16   |  911  | 1049           |             786 ms |
| Q24   |   48  | 55             |              56 ms |
| Q26   |   41  | 45             |              45 ms |

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: adriangb