test(datastore): cover the container upstream associate against Postgres part 6/15 (S17 Phase 6 plan: 42/42)

Why

Parts 1 to 5 built the store with unit tests over its pure functions and statement shapes. None of those reaches Postgres, so the deferred position key, the renumbering under a real unique constraint, and the family rules are unproven until this suite runs them.

The stack

Fifteen parts, each stacked on the last, each green on its own, reviewed in this order. Each part stays under 700 changed lines, counted as insertions plus deletions against the part it targets.

Part	MR	What
1	feat(datastore): container virtual upstream sto... (!2778 - merged) • Hayley Swimelar • 19.5	store types and the list read
2	chore(datastore): add the container upstream po... (!2839 - merged) • Hayley Swimelar • 19.5	the position helpers
3	chore(datastore): add the container upstream wr... (!2841 - merged) • Hayley Swimelar • 19.5	the write statements
4	feat(datastore): the container upstream write e... (!2843 - merged) • Hayley Swimelar • 19.5	the write envelope and associate
5	feat(datastore): container upstream reposition ... (!2844 - merged) • Hayley Swimelar • 19.5	reposition and dissociate
6	test(datastore): cover the container upstream a... (!2846 - merged) • Hayley Swimelar • 19.5	associate against Postgres
7	test(datastore): cover the container upstream m... (!2852 - merged) • Hayley Swimelar • 19.5	the mutations against Postgres
8	test(datastore): cover the container upstream l... (!2853 - merged) • Hayley Swimelar • 19.5	the list read and repairs against Postgres
9	feat(datastore): refuse deleting a container vi... (!2854 - merged) • Hayley Swimelar • 19.5	the repository delete guard
10	Draft: chore(managementapi): empty part 10/15, ... (!2855 - closed) • Hayley Swimelar • 19.5	closed unmerged, its core landed with Step 41
11	feat(managementapi): container virtual upstream... (!2856 - merged) • Hayley Swimelar • 19.5	the association routes
12	test(managementapi): cover the container upstre... (!2857 - merged) • Hayley Swimelar • 19.5	handler reads and associate
13	test(managementapi): cover the container upstre... (!2858 - merged) • Hayley Swimelar • 19.5	handler refusals and update
14	test(managementapi): cover the container upstre... (!2859 - merged) • Hayley Swimelar • 19.5	handler dissociate and the catalogs
15	test(managementapi): the container upstream ass... (!2860 - merged) • Hayley Swimelar • 19.5	the integration walk

The delete guard in part 9 lands before the routes in part 11, so no window exists where a container association is writable without the refusal that protects it.

Related to #314 (closed)