fix(engine): term/terms on keyword ARRAYS match only the first element - #40

Problem

A reverse call-graph query — "who calls wp_safe_remote_get" — returned 1
result from XERJ's structured term index, while grep found 14 files and XERJ
full-text (match) found 9. A false "not reachable" is the worst error in a
security audit.

Root cause (proven)

A keyword field stores one value per document, and for an array value XERJ
keeps only element [0]:

• Memtable DocValues::push_field, the Value::Array arm, does kcol.push(first_str).
• Segment KeywordColumn.ords: Vec<u32> is one ordinal per doc — structurally single-valued.

Aggregations use a separate multi-valued path and FTS joins all elements, so
terms-aggs and match see every element but exact term sees only [0]. This
breaks ES multi-valued keyword semantics.

Reproduced end-to-end (calls: ["first_fn","second_fn","wp_safe_remote_get"]):

term calls=first_fn           -> 1   ([0])
term calls=second_fn          -> 0   ([1] MISSED)
term calls=wp_safe_remote_get -> 0   ([2] MISSED)

Fix

The always-correct source scan already handles arrays (json_values_equal does
array membership), so route term/terms on array fields to it:

• This PR (memtable half): doc_values_term_query, doc_values_term_indices,
  and doc_values_terms_query bail (return None) for fields in array_fields,
  mirroring the existing keyword_has_whitespace bail. Compiles clean; no regressions
  in the touched crate.
• Follow-up (segment half): guarantee the flush multi_valued set covers every
  ingest path and route the remaining fused-columnar term path to the scan (or
  make KeywordColumn multi-valued). A regression test
  test_term_matches_non_first_array_element is included, #[ignore]d until then.

Also found (filed for the series)

Boolean term matches as a string: {term:{f:true}} → 0 while {term:{f:"true"}}
matches. JSON booleans must coerce to the stored "true"/"false" form.

Impact (measured, real WP index of 11,990 functions)

One reverse-reachability query:

• scroll whole graph to client + traverse in code: ~482,800 tokens
• grep + read to confirm: ~4,300 tokens
• fixed: one terms query → exact callers as facts: ~170 tokens (2,823× / 25×)

Restores composable structured graph queries: audit cost goes from O(index size)
to O(matching results), and makes the pre-AST index strictly better than grep for
reachability. Full write-up: docs/research/xerj-keyword-array-term-fix.md.

Files

• engine/crates/xerj-engine/src/memtable.rs — array-field bail in 3 fast paths
• engine/crates/xerj-engine/tests/integration.rs — reproduction test (#[ignore])
• docs/research/xerj-keyword-array-term-fix.md — root cause + token math

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