Implement scalar-indexed joins on v2 merge_insert path

[wjones127]

What to build

Implement the indexed join approach chosen in #6443 so that upserts with a
scalar index on the join key use the v2 path instead of falling back to v1.

Remove the scalar index fallback from can_use_create_plan. The use_index
parameter is retained as an escape hatch that controls whether the indexed
join node is used in the plan.

See milestone description for full PRD.

Acceptance criteria

• Upserts with scalar-indexed join keys use the v2 path (verified via explain_plan)
• Existing indexed merge insert tests pass unchanged
• New explain_plan test confirms indexed join node appears in plan
• use_index=false still works as escape hatch (bypasses index, uses full scan join)
• Conflict detection (Bloom filter) works for indexed operations

Blocked by

• Research spike: indexed join strategy for v2 merge_insert #6443 (research spike must complete first)

User stories addressed

• 3: indexed upserts use v2 path
• 4: one fewer case on v1, moving toward single code path
• 5: use_index retained as escape hatch