Order a branch's plans by its merges, and build the newest commit (hq issue 349)

A merge acted on late by the catch-up made its plan after the plan of the
merge that followed it, superseded it by creation time, and folded its
unbuilt modules into a plan at the older commit: on 2026-10-09 the
forge's security fix (a082615b) was superseded by 8ff8197a. A plan now
keeps its merge time (migration 0086), supersession follows it, and a
merge older than an open plan of its branch is planned at that plan's
commit, which contains it.

internal/inventory/migrations/0086-a-plan-keeps-when-its-merge-was-made.sql

@@ -0,0 +1,10 @@
+-- A plan keeps when the forge made the merge it answers (novox/hq issue 349).
+--
+-- A newer plan of a repository's branch supersedes the older open ones, and "newer" was read from when each
+-- plan was made. A merge the bus did not hand over is acted on late, by the catch-up, so its plan is made
+-- after the plan of a merge that came after it — and superseded it, folding its unbuilt modules into a plan
+-- at the older commit. On 2026-10-09 a security fix to the forge's module would have been built from the
+-- commit before it. Merges into one branch are made one after another, each on the one before, so the
+-- forge's merge time is the branch's order. Null for a plan kept before this column, and for a plan no merge
+-- made (a release); then the plans' own order stands, as before.
+alter table release_plan add column merged_at timestamptz;