fix(merge-insert): apply Delete/Fail on the indexed-scan path

[hamersaw]

Problem

An indexed merge-insert delete by a composite primary key whose columns are all indexed silently removes nothing. merge_insert(when_matched = Delete, use_index = true) reports deleted = 0, the matched rows stay live in the table, and resurface in later reads.

Root cause

WhenMatched::Delete is only implemented in the v2 plan (DeleteOnlyMergeInsertExec), which never uses a scalar index. When every join column is indexed, can_use_create_plan routes the merge to the legacy Merger instead (the only engine with the indexed-scan probe). That engine's matched-row handler only ever distinguished DoNothing from "update" — it folded both Delete and Fail into the update path:

• a fully-indexed Delete rewrote the matched rows in place → 0 deletes;
• a fully-indexed Fail silently updated instead of erroring.

A single indexed column hits the same path, so this was never composite-specific — composite keys just make it the common case (partial-column updates require an index on every PK column).

Fix

Make the legacy Merger dispatch every WhenMatched variant explicitly, mirroring the v2 classifier (merge_insert_action):

• Delete collects matched row ids as deletions and emits no replacement batch.
• Fail aborts on any match, with the same message as the v2 path.
• A delete-only commit branch drains the merger and applies the deletions (resolving row ids → addresses via the row-id index for stable row ids) without writing fragments — this keeps the O(keys) indexed delete instead of falling back to a full table scan.
• The partial-schema (column-rewrite) commit branch physically cannot express row deletions, so combining Delete with inserts from a partial-schema source now returns a descriptive error rather than silently dropping the deletes.

Tests

cargo test -p lance --lib dataset::write::merge_insert (152 passing), covering:

• composite-key indexed delete (index on first / second / both / neither — the both-indexed case is the original bug);
• single-column indexed delete;
• multi-fragment + stable-row-id variants, including an appended fragment neither index covers (unindexed-remainder union);
• Delete combined with InsertAll (not delete-only);
• Fail on an indexed key (match aborts, no-match inserts cleanly).

cargo fmt and cargo clippy -p lance --lib --tests -- -D warnings are clean.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 89.80716% with 37 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
rust/lance/src/dataset/write/merge_insert.rs	88.88%	30 Missing and 7 partials ⚠️

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