Use a bounded 512-byte stack fallback for complete-snapshot record
scratch. Small records avoid heap growth while larger records continue
through the heap allocator.
This usually means 2 fewer allocations per encode, resulting in a small
performance improvement in a local benchmark:
| Workload | `main` median | Branch median | Change |
|---|---:|---:|---:|
| Empty, 50k encodes | 135.06 ms | 130.76 ms | **3.2% faster** |
| 1 MiB ASCII, 500 encodes | 280.75 ms | 281.25 ms | **0.2% slower**,
within noise |
| 1 MiB styled, 200 encodes | 1,023.02 ms | 996.32 ms | **2.6% faster**
|
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