Add the chain package and its buckets: builds, rollouts and assignments with history 64 (issue 1089)

internal/chain holds typed Build, Rollout and Assignment and a Backend with no delete or purge, in memory and on the bus. The builds and rollouts buckets are asserted with history 64, no age and values up to just under a mebibyte; the assignments bucket gets the same, keeping its marker age because a stream cannot be made to disallow message ages. Migration 0104 makes the one-row chain_moved switch. The controller keeps no consolidated schema, so 0104 is the whole schema change. Nothing reads the buckets yet.

internal/inventory/migrations/0104-the-chain-moves-to-the-bus.sql

@@ -0,0 +1,16 @@
+-- Builds, rollouts and assignments move to the bus (novox/hq ADR 0408 §8, design 47 milestone 5, issue 1089).
+--
+-- This migration makes the switch alone. The rows of build, build_request, release_plan, batched_merge, assignment and
+-- pending_assignment are written into the chain's buckets by the serving controller at its start, holding the lease,
+-- because a migration runs when the store is opened, before any connection to the bus. This row is the switch: while it
+-- is absent nothing reads the buckets; once the move has counted every row against the keys it wrote, it is written, and
+-- nothing reads the tables. One row at most; never written by a migration, so a delivery alone changes nothing. The
+-- controller keeps no consolidated schema, so this migration is the whole of the change; the tables go by a later one.
+create table chain_moved (
+    only_one boolean     primary key default true check (only_one),
+    moved_at timestamptz not null default now(),
+    -- The counts the move read from the tables and wrote as keys, per kind, and the largest value it wrote: what a
+    -- reader of the move reads back.
+    counts   jsonb       not null,
+    largest  jsonb       not null
+);