icons: per-platform icon set — real Liquid Glass on macOS, flat design on Windows/Linux

The hand-packed icns was pixel-clean (verified by unpacking and scanning
every size), but macOS 26 composites legacy .icns icons into the Liquid
Glass system at render time; stacked on Icon Composer's baked PNG-export
lighting, that clips the rim highlights into the near-corner specks and
abrupt highlight ends (owner report 2026-10-13). The fix is the real
layered icon: mac.icon now points at the Icon Composer document
(build/icon.icon, copied from ~/Documents/Affinity/lh-logo) and
electron-builder compiles it via actool into Assets.car — macOS 26+
renders the true Liquid Glass design (dark/tinted variants included).

actool's backwards-compat icns caps at 256px, so scripts/after-pack.cjs
swaps in the existing full-res build/icon.icns before signing: pre-Tahoe
macOS keeps today's quality, Tahoe gets the layered icon.

Windows/Linux get the owner's flat design (rounded squircle with baked
transparency): scripts/make-icons.mjs renders build/icon-flat.svg via
resvg into build/icon.ico (16→256 PNG entries) + build/icon.png (512),
and electron-builder.yml pins win.icon/linux.icon.

desktop:dist injects DEVELOPER_DIR (actool needs Xcode; the machine
selects CommandLineTools by default, CI's setup-xcode takes precedence
via the :- default). The release workflow's mac job selects Xcode 26+
explicitly. Verified locally: mac --dir build (Assets.car + checksummed
full-res fallback + CFBundleIconName/IconFile) and the win-unpacked exe
carries our ico bytes; NSIS/AppImage assembly is CI's (cross-build
tooling can't run on arm64 macOS). Runbook updated. 1610/1610.

electron-builder.yml

@@ -13,6 +13,19 @@ appId: com.learnharness.LearnHarness
 productName: LearnHarness
 directories:
   output: dist
+# Icons (owner ruling, 2026-10-13) — per-platform sources in build/:
+#   mac   build/icon.icon  — the Icon Composer document (layered Liquid
+#         Glass); actool (Xcode 26+) compiles Assets.car (CFBundleIconName,
+#         macOS 26+ renders the true layered icon) + an actool fallback icns
+#         capped at 256px, which scripts/after-pack.cjs swaps for the
+#         hand-packed full-res build/icon.icns (pre-Tahoe keeps 16→1024).
+#         Requires actool ≥ 26: desktop:dist injects DEVELOPER_DIR locally;
+#         the release workflow's mac job runs setup-xcode.
+#   win   build/icon.ico  — generated (16→256) from the flat design by
+#         scripts/make-icons.mjs.
+#   linux build/icon.png  — 512×512, same script. Rounded squircle with
+#         baked transparency on both (owner ruling) — Windows/Linux don't
+#         mask, the shape ships in the artwork.
 files:
   # Only the shell goes into the asar. ORDER IS LOAD-BEARING (payload
   # wave, 2026-09-24): electron-builder's dependency collector packs the
@@ -82,6 +95,7 @@ extraResources:
     filter:
       - "standalone/**/*"
 mac:
+  icon: build/icon.icon
   # zip rides next to dmg (flipped 2026-10-08): Squirrel.Mac — Electron's
   # macOS auto-updater — installs from ZIP and cannot read DMGs. DMG is
   # for humans; ZIP + blockmap are the updater's payload.
@@ -114,7 +128,9 @@ mac:
   notarize: true
 dmg:
   title: "${productName} ${version}"
+afterPack: scripts/after-pack.cjs
 win:
+  icon: build/icon.ico
   target:
     - target: nsis
       arch:
@@ -128,6 +144,7 @@ nsis:
   oneClick: true
   perMachine: false
 linux:
+  icon: build/icon.png
   target:
     - target: AppImage
       arch: