Closeout: record the M3.10a merge facts and repair the kernel-target statement

[kaikisegfault]

Bounded closeout for the concluded M3.10a slice (#169, PR #170, merged). No new slice work.

docs/project/current-state.md could not carry the merge facts before the delivery PR merged, and its "What works now" section still named economy-transition-v5 as the surface the C++ kernel must be implemented against — which version six superseded on the same day.

Scope

• Record PR economy-transition-v6: the identity is the account #170's merged commit range and the exact candidate and post-merge run results.
• Replace the stale kernel-target statement with a version-six entry, and restate version five accurately as accepted, evidenced, and superseded.

Documentation only. The next slice, M3.10b (#153), is not started.