Seats keep their former names, so a rename breaks nothing (ADR 0122, phase 2)

Phase 1 made the set data; a rename still broke every reference to the old
name. This adds the stable identity: a seat's canonical name changes and its
old name becomes an alias that resolves to it forever. SeatNamed and the holder
and display matching resolve a name (former or current) to its seat, so a
manifest's claim, a held record, the git-seat lookup and the build machine's
embedded set all go on working unchanged after a rename. seat_alias table
(migration 0035), inventory Aliases/RenameSeat, openInventory loads them, and a
'seat rename <from> <to>' command does the whole thing — one operation, no
rebuild, no re-registration, no freeze. Behaviour-neutral until a seat is
renamed. Validated against postgres.

internal/inventory/migrations/0035-a-seat-keeps-its-former-names.sql

@@ -0,0 +1,12 @@
+-- A seat keeps its former names, so a rename breaks nothing (novox/hq ADR 0122).
+--
+-- Phase 1 made the seat set data, but a rename still broke every reference to the old name — a
+-- manifest's claim, a held record, the git-seat lookup — because they name the seat and the name
+-- had changed. This is the stable identity ADR 0122 asked for, realised the simple way: a seat's
+-- canonical name changes, and its old name becomes an alias that resolves to it forever. Nothing
+-- downstream has to change — a manifest goes on claiming the old name, the build machine goes on
+-- validating it — and a rename is one operation: set the new name, remember the old.
+create table seat_alias (
+  alias text primary key,       -- a former name of a seat
+  seat  text not null           -- the seat's current canonical name it resolves to
+);