Adds the three driver commands, parameterized E11000 messages (engine
dup_index carries the index name into writeErrors), the scan_matching
planner wiring (_id fast path → index plan → scan) and the first-$match
aggregate pushdown. The equivalence test (mixed-type corpus x 27 filters,
non-sparse and sparse indexes) drove out three real bugs: the two-bound
range under-approximation on multikey indexes (fall back to scan), a
dangling single-value option array in the planner, and update-time
unique violations now reporting writeErrors instead of corrupting state.
Collection gains an indexes list; evict_doc removes entries at the single
document-death chokepoint; upsert does build → check → reserve → log →
evict → publish so entry insertion after the append is infallible and a
rejected unique write never reaches the log. Replay registers empty
indexes from create/drop records (types 3/4) and Engine.open rebuilds them
from live docs. compact re-emits index-create records. Engine gains
create_index/drop_index and dup_index for E11000 naming.
Adds src/index.zig with the full secondary-index machinery: entry
generation mirroring field_matches (array value + elements), BSON-order
sorted entries with binary search, compound prefix and range lookups,
unique/sparse options, the query planner (longest equality/$in run +
optional range, $in cartesian cap, sparse/null bail), and the _id_ fast
path guarded against serialization-ambiguous values (numbers, strings,
symbols, codes, opaque payloads).
query.collect_values is now pub so entry generation can mirror it exactly.
storage.zig gains record_type_index_create/drop; lib.zig exports index.
