Materialize the trash — storage layer

Phase 1 of the trash pivot: the file format learns .trash/. The loader
parses the reserved root container — cards only, one shared parseCard
for both containers so fail-fast, attachments, and verbatim documents
are literally the same code; absent means empty; symlinks and
lane-shaped nestings fall out as strays by construction. BoardModel
grows snapshot.trash as a plain rank-ordered card list — the container
has no identity to carry. Legacy deleted: keys keep flowing through
the retiring flag path so every tombstone consumer stays green, and
are additionally reported through LoadResult.legacyTombstones in the
loose-file idiom for phase 2's migration scheduling — nothing vanishes
from view before its folder has actually moved, which is also 01's
lock-deferral posture. Writer primitives land value-passing: move to
trash with caller-minted rank and the deliberate modified stamp,
tombstone migrations that surgically remove the key, physical lane
removal, per-card and whole-container purge that leaves strays
verbatim, and byte-faithful whole-subtree capture/recreate for lane
undo. Board-wide identity now spans the trash, so an import colliding
with a trashed UUID remints instead of colliding. The watcher already
delivered .trash events — isGitInternal tests a component, not a dot —
now stated and pinned rather than relied on.

Claude-Session: https://claude.ai/code/session_01SR4XGjmBE16ZUYWpfFHXwY

Kanban/LiveStore/BannerCenter.swift

@@ -635,6 +635,13 @@ public final class BannerCenter {
             if let title { "Couldn't put '\(title)' back" } else { "Couldn't put the item back" }
         case let .purge(title):
             if let title { "Couldn't permanently delete '\(title)'" } else { "Couldn't permanently delete the item" }
+        case let .migrateTombstone(title):
+            // One sentence covering both shapes of the migration, because the user's mental model
+            // of either is the same non-event: a board written by an older version being brought
+            // up to date. It deliberately names neither "delete" nor "trash" — a card's migration
+            // moves it into the trash and a lane's brings it *back*, so any verb specific enough
+            // to describe one would be actively wrong about the other.
+            if let title { "Couldn't update '\(title)' to the current format" } else { "Couldn't update an item to the current format" }
         case let .style(title):
             if let title { "Couldn't restyle '\(title)'" } else { "Couldn't restyle the item" }
         case let .resize(title):