Pin Nucleus v1.2.1 (upstream fix for the L9 ALTER corruption)

[im-tyler]

Nucleus v1.2.1 shipped today with the upstream fix behind L9: ALTER TABLE ADD/DROP COLUMN and ALTER COLUMN TYPE are now all-or-nothing on the disk stack (layout adopted, every row rewritten, indexes rebuilt, every tuple verified to decode; any failure restores the pre-change state), and half-applied ALTERs are no longer recorded as migrated — the state that left live llm_traces undecodable with token_source present and cost_source missing after migration 054.

Changes:

• teploy.yml: pin ghcr.io/neutron-build/nucleus:v1.2.1 with the dated changelog comment (convention), including the pointer to the in-place catalog repair the infra-home store still needs before 054 can re-run.
• ci.yml: new NUCLEUS_VERSION workflow env (single bump point); both integration jobs now reference it; step names de-versioned. Same shape the S1 branch introduced, so that merge converges to one line.
• docker-compose.yml: pinned (was :latest) via ${NUCLEUS_VERSION:-v1.2.1}.
• AUDIT_OPEN.md: L9 updated — upstream resolved; (b) no longer needed for correctness on v1.2.1+ (the engine refuses ALTER-after-write orderings itself); (c) covered by the S1 populated-upgrade test; (a) remains an owner op, now with the row-preserving in-place repair instead of rename-aside loss.

Verified before bumping, exact CI shape (fresh v1.2.1 container, migration boot 001-056, then go test -p 1 ./... with OBSERVE_REQUIRE_NUCLEUS=1): 48 packages, all green.

Migration files intentionally unchanged — ADD COLUMN IF NOT EXISTS is safe again on this engine per the upstream handover (nucleus/docs/HANDOVER_TEPLOY_OBSERVE_ADD_COLUMN.md).