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/App/AppModel.swift

@@ -634,6 +634,12 @@ public final class AppModel {
     /// not validity, so a malformed `deleted:` counts as deleted here exactly as it does everywhere
     /// else.
     ///
+    /// **`.trash/` is excluded by construction** (02-architecture.md § Per-board app state,
+    /// re-grounded 2026-07-28 for the materialized trash): this walks `snapshot.lanes`, and the
+    /// trash is `snapshot.trash` — a sibling container, never a lane — so no filter is needed and
+    /// none could be forgotten. The flag walk above is the retiring half of the same rule, kept
+    /// while boards written by older versions still carry `deleted:` keys.
+    ///
     /// Static and pure: it is a fact about a snapshot, and the close flush is the wrong place to
     /// discover a counting bug.
     public static func liveCounts(of snapshot: BoardModel) -> (lanes: Int, cards: Int) {