v1.0.1: cycle FinderInfo kHasCustomIcon flag on every apply

Force a clear -> set transition of the kHasCustomIcon FinderInfo
flag on every apply (was previously just |= the bit, which on
overwrites wrote the same byte and didn't trigger any state change
Finder could observe).

Empirically does not, on its own, force Finder to refresh stale
cached volume icons -- which was the original motivation, ultimately
addressed by removing the zoom slider in 1eaa148 (the surface that
exposed the cache problem most often). But the cycle is correct
hygiene: first applies degrade to a plain set, overwrites get a
clean state transition, other FinderInfo bytes (colour labels, etc.)
are preserved.

Kept in reserve for any future macOS release where this turns out
to be the missing piece. Documented in decisions.md
("Finder cache invalidation, round 3").

NOTE: This change was already present in the v1.0.1 notarized .app
exported earlier (built locally after the edit, before being
committed). Committing now so the repo matches the shipped binary
before the tag moves to point here.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Xpycode

01_Project/Sigil/Services/IconApplier.swift

@@ -92,9 +92,15 @@ actor IconApplier {
             throw Error.underlying(error)
         }
 
-        // Step 2: read-modify-write FinderInfo to set byte 8 = 0x04.
+        // Step 2: cycle the FinderInfo `kHasCustomIcon` flag (clear → set) so
+        // Finder sees a state transition, not just a same-bit re-write.
+        // First applies (flag not yet set) degrade to a plain set; overwrites
+        // (flag already set from a previous apply) get the toggle, which
+        // empirically punches through Finder's icon-services cache when
+        // `noteFileSystemChanged` alone wasn't enough. See decision log
+        // 2026-05-04 ("Finder cache invalidation, round 3").
         do {
-            try Self.setCustomIconFlag(on: volumeURL)
+            try Self.cycleCustomIconFlag(on: volumeURL)
         } catch {
             // Roll back the orphan icon file — otherwise we'd leave garbage.
             try? fm.removeItem(at: iconURL)
@@ -154,6 +160,21 @@ actor IconApplier {
 
     // MARK: - Private helpers
 
+    /// Force a `kHasCustomIcon` state transition by clearing then setting
+    /// the flag. Bypasses Finder's icon-services cache, which on overwrites
+    /// otherwise sees "flag still set, no change" and serves the stale
+    /// cached icon.
+    ///
+    /// Safe with respect to other FinderInfo bytes (Finder colour labels
+    /// etc.): the underlying `clearCustomIconFlag` only touches byte 8 and
+    /// preserves the rest of the 32-byte blob; if the rest is all-zero, it
+    /// removes the xattr entirely (cleanest "fresh state" signal to
+    /// Finder), and `setCustomIconFlag` re-creates it with just byte 8 set.
+    private static func cycleCustomIconFlag(on volumeURL: URL) throws {
+        try clearCustomIconFlag(on: volumeURL)
+        try setCustomIconFlag(on: volumeURL)
+    }
+
     private static func setCustomIconFlag(on volumeURL: URL) throws {
         var info = (try XAttr.get(name: finderInfoKey, from: volumeURL.path)) ?? Data()
         if info.count < finderInfoLength {