Add headless regression tests for Prop Editor functionality

- Implement tests for PropFile, including preset conversion, file dictionary conversion, round-trip serialization, version rejection, default values for missing keys, flattening, sanitization, and save/load functionality.
- Create tests for PropEditor, covering window title formatting, mode button text, safe saving, and recent file list management.
- Add tests for PropTheme, validating fallback behavior, invalid input handling, and default values for primitives and UI tokens.
- Introduce tests for ThumbnailCache, ensuring proper key formatting, PNG handling, and behavior with missing files.

specs/phase_9_prop_editor_round5.md

@@ -0,0 +1,49 @@
+# Phase 9 Prop Editor — Round 5 (File → Recent + save-serialization fix)
+
+Status: **implemented** (2026-09-19; direct bugfix + feature round).
+
+Source: two user feedback points against the shipped Round 4 Prop Editor.
+
+## Feedback summary
+
+1. File → Recent: show the last 5 opened props (sandbox-stage pattern).
+2. Loading `props/eye.prop` draws nothing (the prop name shows, geometry is blank).
+
+## Diagnosis (point 2 — root cause)
+
+`PropFile._serialize_shape()` declared
+
+```gdscript
+var points: PackedVector2Array = shape.get("points", PackedVector2Array())
+```
+
+but the editor always serializes **export-format** shapes whose `points` is an `Array` of
+`{x, y}` dictionaries. The implicit `Array → PackedVector2Array` conversion cannot construct
+`Vector2` from a dictionary and **silently yields `Vector2.ZERO` per element**, preserving the
+count but zeroing every coordinate. Every save therefore wrote all points as `(0, 0)` —
+verified in `props/eye.prop` and `props/box.prop` (all `x: 0.0, y: 0.0`), which load as a
+degenerate, invisible shape. The round-1 tests only asserted point *counts* and dict *shape*,
+so the corruption slipped through.
+
+**Fix:** `_serialize_shape()` now uses the existing `_coerce_points()` helper (which handles
+both `PackedVector2Array` and `Array`-of-`{x,y}` inputs). Regression assertions were added so
+point **values** are verified through `to_file_dict` and the full round-trip.
+
+The two already-saved user files (`props/eye.prop`, `props/box.prop`) are unrecoverable
+(every coordinate was zeroed at save time) and must be re-authored.
+
+## Changes
+
+| Target | Change |
+| --- | --- |
+| `scripts/prop_file.gd` | `_serialize_shape` uses `_coerce_points` (bug fix). |
+| `scripts/prop_editor.gd` | `SETTINGS_PATH := "user://prop_editor_settings.json"`, `RECENT_MAX := 5`, `_recent_props`; `update_recent_list(list, path, max)` static (dedupe + front-insert + trim); 🕑 Recent submenu in the File menu (`RecentProps` child popup, updated on `about_to_popup`, missing files pruned, "(No recent props)" placeholder); `_load_settings`/`_save_settings`; `_push_recent(path)` on every successful load; missing recent → error dialog + removal. |
+| `tests/test_prop_editor_file.gd` | New value-preservation assertions (serialized `x == 64`, round-trip `(64,0)`/`(64,64)`). |
+| `tests/test_prop_editor_round3.gd` | New `_test_recent_list` (dedupe, front-insert, trim). |
+
+## Verification
+
+- `--check-only` clean; all five suites green: file 66, theme 53, canvas 87, thumb 10, round3 21.
+- Manual F6: save a prop with shapes at distinct coordinates, reload — geometry must match;
+  File → 🕑 Recent lists the last 5 opens and opens them; missing recent file shows the error
+  and prunes itself.