Keep editing while a project saves; saves, reloads and opens queue behind one writer (12.13b)

TASKS.md

@@ -387,7 +387,7 @@ The panel of 4.6 shows and edits what the session holds so far. Each item adds i
 - [x] 12.12c1 Part three: numeric label scrubbing (`2d36d6e`, `680a522`) in the tool bars and the layer opacity; the build-warning fixes (`075a71b`, `725b934`) where they change behaviour. Codex review: 3 rounds. Round 1: focused fields left stale, double-click second presses, Feather's amount wrongly resting, test gaps. Round 2: an opacity scrub outliving its layer, boundary gaps. Round 3: the same for the Move bar. All fixed; closed after the third.
 - [x] 12.12c2 Numeric label scrubbing in the sheets and panels: Canvas Size, Image Size, the colour picker, effects, filters, Hue/Saturation, Levels, Camera Raw. Codex review: 3 rounds. Round 1: every Mixer row scrubbed where Swift scrubs the HSL families alone, and test gaps (Image Size limits, Camera Raw bindings, filter decimals, centimetres). Round 2: test gaps (a held drag across `reshape`, titles enabled again, Alt-held Camera Raw scrubs). Round 3 passed.
 - [x] 12.13a Version 1.3: reloading a project changed on disk (`ed5ddea`, merged in 1.3), its manifest-and-sizes fingerprint (`cd4061a`), images decoded from memory (`cba6efc`). Codex review: 3 rounds. Round 1: a package exchanged and kept aside unseen (Qt watches no IN_MOVE_SELF), a failed manifest read still digested, coarse timers, a failed save's guard released early, and test gaps. Round 2: test gaps (the watch moving, a second change while asking, quiet-time and re-arm counts, an ordered test for a vanished image). Round 3 passed.
-- [ ] 12.13b Keep editing while a project saves (`940e188`).
+- [x] 12.13b Keep editing while a project saves (`940e188`). Codex review: 3 rounds. Round 1: overlapping writes and a quit before a write finished, a queued save crashing after Undo took the document, a reopen reading a package mid-save, and test gaps. Round 2: an external reload reading mid-save, and test gaps. Round 3: a failed save stranding an external change already seen, and test gaps. All fixed; closed after the third.
 - [ ] 12.13c The guide for AI agents writing projects (`c317306`), README's lines.
 - [ ] 12.14 Versions 1.3.1–1.3.2: quitting and closing while text is edited, the I-beam over the canvas, File > Open Recent, Image Size keeping print sizes, Camera Raw's Color Grading layout, coloured slider tracks with double-click reset, colouring only the selected letters, sampling keeping a panel's focus, filters waiting for text editing to finish.
 - [ ] 12.15 Version 1.3.3: the Dither filter.