test(core): pin that append-only reads do not apply deletes

[linliu-code]

Independent of the merge-on-read reader port — this is about the existing read path.

What I found

RecordMerger's append-only arm is:

RecordMergeStrategyValue::AppendOnly => record_batches.concat_data_batches(self.schema.clone())

concat_data_batches concatenates data_batches and returns. delete_batches is never read. A deleted record is still in the result.

Why that is reachable

Append-only is defensible for a table that genuinely only appends. But it is not opt-in — config/table.rs derives it:

• hoodie.populate.meta.fields = false → append_only, or
• no hoodie.table.ordering.fields / precombine.field → append_only

A merge-on-read table meeting either condition has log files that can carry delete blocks. Those deletes are dropped, and updates are returned as duplicates rather than merged (the existing test_merge_records_append_only already documents the duplication — 4 rows out of 2+2 with a repeated key).

Why it went unnoticed

I checked every merge-on-read fixture in the repo: all 14 set an ordering field and leave meta fields on, so none derives append-only. The path has no fixture coverage at all.

What this PR does

Adds a test pinning the current behavior, so a change to it is deliberate rather than accidental. It does not change behavior — I did not want to alter existing read results in a test-only PR, and the right fix is a design question:

• treat a merge-on-read table with delete blocks as never append-only, or
• apply deletes on the append-only path too, or
• reject the combination loudly

Worth a maintainer's call. This came up because the ported merge-on-read reader has no append-only equivalent — it always merges by key — so the same table would return different rows under it.

🤖 Generated with Claude Code

[linliu-code]

Closing: already contained in the surviving line.

test_append_only_does_not_apply_deletes is the one difference between the old
#639-#662 tip and the squashed commit the current line starts from, so it is
present in #673 today (crates/core/src/merge/record_merger.rs). Keeping this
open would put the same test in two open PRs.

Review it in #673, or in #660 for the whole line squashed.