Memoize the reload parse and short-circuit value-equal snapshots

The loader gains a ParseMemo — the previous walk's parsed documents keyed
by root-relative path, trusted on the git-index heuristic (mtime + size,
no hashing) and passed as an input so the loader stays stateless. A hit
skips exactly one file read; schema, order, coercions, dedupe, and every
directory listing run fresh, so memoized and cold walks are output-
identical (golden-corpus equivalence suite). Entries record only past the
schema gate, so a defect can never be answered from the memo.

The store skips the snapshot assignment wholesale when the fresh model is
value-equal — no @Observable churn, no render pass, no snapshotGeneration
bump — and a new landedReloads counter carries walk-completion for the
three consumers whose subject is the walk, not the snapshot: the card
window's comment thread, the comment search index, and the auto-committer's
covering gate (which now counts a completed walk as covering even when
nothing changed). Warnings and defects move on their own equality; failed
reloads bump neither counter. An injectable ParseCounter makes the
single-file-echo claim a test.

Claude-Session: https://claude.ai/code/session_01CqjXB7ASoWtbyoGod68k97

Kanban/UI/Board/DragSession.swift

@@ -162,6 +162,13 @@ struct FileDropTarget: Equatable, Sendable {
 /// app-mediated echo is normally next, and a foreign one that lands first re-grounds everything
 /// anyway.
 ///
+/// It watches `BoardStore.snapshotGeneration`, which since 2026-07-31 does not move for a reload whose
+/// tree came back value-equal, and that is the right counter rather than a hazard: a hold is only ever
+/// armed by a drop whose write actually rearranged something — every drop path refuses a no-op
+/// arrangement *before* it opens a write bracket (`BoardStore.moveCards`, `moveLanes`, `moveLane`) —
+/// so a drop that would land value-equal never writes and never reloads, and its hold is the
+/// watchdog's to retire exactly as it was before the skip existed.
+///
 /// The `timeout` is the same guarantee the drag session's watchdog gives the drag itself: a write
 /// that was refused outright (a read-only board) produces no reload at all, and an overlay with no
 /// hand-off coming must still dissolve and let the snapshot be the authority again.