chore(beads): add 8tgic — completion-debt for missing SIMD dot product tests

Tracks a [completion-debt] gap surfaced by the audit pass: bead
ylnl was closed with a `close_reason` claiming SIMD dot product
tests had been added, but the corresponding test file was never
written. New bead 8tgic carries the work forward (write the
SIMD dot product tests, including a parity check against the
scalar implementation and a fallback path for non-SIMD targets).

Co-Authored-By: Claude <noreply@anthropic.com>

Author: Dicklesworthstone