Fuzz harness: cache-config sweep with config-invariance and cache-state oracles

[zfarrell]

Part of #22. Depends on the harness skeleton.

Two oracles that need no reference engine at all: the cache must not change the answer, whatever its configuration or its state.

Config invariance

The same SQL over the same file, run under N cache configurations, must produce identical results. Dimensions:

• Memory limit, including values small enough to force eviction mid-scan
• Disk limit at 0, small, and large — 0 disables the liquid disk tier entirely
• All three squeeze policies (Evict, TranscodeEvict, TranscodeSqueezeEvict)
• Cache batch size deliberately mismatched against session batch size. This is the Reader batch id counts query batches but addresses cache batches #13/fix(reader): read the cache in cache-sized batches #15 dimension. The builder pins both to the same value (src/datafusion-local/src/lib.rs:186), so the mismatch is produced by SET datafusion.execution.batch_size after build, exactly as batch_size_alignment.rs:58-66 does.

test_runner (tests/mod.rs:170) already does a 3x3 version of this over 6 hand-written queries; this generalizes it to generated queries and adds the batch-size axis.

Cache state invariance

Within one configuration, the same query must return the same rows:

1. Cold — nothing cached
2. Warm — immediately after, cache populated
3. After forced eviction — exercises the squeeze and hydration paths, which no current test reaches through a generated workload

run_sql_with_cache (tests/mod.rs:130) already asserts cold == warm for its six queries; the third run is new.

Why this matters independent of the differential oracle

These catch bugs where vanilla DataFusion is unavailable as a reference — unsupported types, forced-option divergence — and they attribute unambiguously: if two cache configurations disagree, exactly one of them is this repo's bug, with no upstream explanation available.

Acceptance

• Reverting the fix(reader): read the cache in cache-sized batches #15 fix makes the harness fail within the default iteration count.
• The failing configuration pair is named in the finding, not just the diff.