Store keys in key order in SST blocks that omit hashes (#97480)
## What
Key blocks for short keys (≤ 32 bytes) don't store a per-entry hash, but
were still sorted by `(hash, key)`. That forced every binary-search
probe to recompute the entry's xxh3 hash just to compare it — roughly
ten hashes per lookup. These blocks now store their entries in **key
order**, so the search compares key bytes directly and hashes nothing.
All the families `turbo-tasks-backend` produces hit this case, so this
is the common path in practice.
## Why this is safe
Hash-based routing is unchanged. Files are still assigned by hash, the
index block still routes to a key block by hash, and
`min_hash`/`max_hash`, the AMQF, and compaction's coverage model all
work exactly as before.
Thus only the order *within* a hash-less block changes. This adds a
small cost to writing, which now must re-sort blocks of keys and a
double cost to compaction which now must re-sort blocks in hash order to
iterate them, and then sort them back to key order when writing the new
file.
## Benchmarks
Measured against `canary`, baseline and comparison run back to back on
the same machine.
**Lookups ** All 16 `static_sorted_file_lookup` configurations improved
| entries | hit/uncached | hit/cached | miss/uncached | miss/cached |
| --- | --- | --- | --- | --- |
| 1 Ki | -30.4% | -24.8% | -29.1% | -29.4% |
| 10 Ki | -25.2% | -19.7% | -22.7% | -23.7% |
| 100 Ki | -17.3% | -19.3% | -15.2% | -16.8% |
| 1000 Ki | -7.0% | -25.7% | -7.0% | -10.9% |
**Commits ** Short keys pay for the added per-block sort.
| config | change |
| --- | --- |
| `key_8` 85Ki entries | +3.3% |
| `key_8` 853Ki entries | +3.1% |
| `key_8` 8.33Mi entries | +8.5% |
| `key_32Ki` / `key_4` (large-key configs) | -2.6% to -19.1% |
Six of thirteen write configurations came out as noise (p >= 0.05) and
are omitted.
**Compaction** `StaticSortedFileIter` must yield `(hash, key)` order
because `MergeIter` merges on it, so it reorders each hash-less block
back into hash order. Plus the additional cost of the commit (above)
| config | change |
| --- | --- |
| 4Mi entries / 8 commits | +6.0% (reproduced at +10.2%) |
| 16Mi entries / 8 commits | +7.5% |
| 4Mi entries / 32 commits | +7.8% |
| 16Mi entries / 32 commits | +5.2% |
| 16Mi entries / 128 commits | -9.5% |
