Save and open projects as JSON

A project is the whole setup as a small text file: the source paths, the
effect chain with its params, the canvas and the seed. Reading and writing
are driven by the effects' ParamSpecs rather than by whatever sits in a
stage's values, so a new effect is saveable without touching this code, a
key an effect does not declare is never written or read back, and an old
project keeps the defaults for params that did not exist when it was
saved. Paths under the project's own folder are written relative, so a
folder of images and their project can be moved whole.

src/core/project.cpp carries its own small JSON reader instead of a
dependency, and names the line when a hand-edited file is wrong.

The interface gets a header-bar menu: New, Open, Save, Save As, with
Ctrl+N/O/S. The title shows the project name. Opening a project replaces
the sources, discarding a decode still in flight rather than mixing it
into the new project.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

CLAUDE.md

@@ -14,11 +14,11 @@ Everything is warning-clean under `-Wall -Wextra -Wpedantic`; keep it that way.
 
 The test suite is a single hand-rolled binary (no framework), so there is no
 per-test filter: run `./build/tests/effector_tests` directly to see the section
-headings (`rng`, `registry`, `scanlines`, `blur`, `chain`, `batch loading`) and the `ok`/`FAIL`
-lines. To iterate on one area, comment out the other blocks in
-`tests/core_tests.cpp` or add a new `std::cout << "section\n"` block — checks are
-plain `check(condition, "description")` calls and `main()` returns the failure
-count.
+headings (`rng`, `registry`, `scanlines`, `blur`, `chain`, `project`, `image
+io`, `batch loading`) and the `ok`/`FAIL` lines. To iterate on one area, comment
+out the other blocks in `tests/core_tests.cpp` or add a new
+`std::cout << "section\n"` block — checks are plain
+`check(condition, "description")` calls and `main()` returns the failure count.
 
 Headless render (no display needed, the fastest end-to-end smoke test):
 
@@ -69,6 +69,22 @@ CLI builds it from repeated `-e`: `-e scanlines --strip-height 6 -e blur
 (`src/cli/options.cpp`) hands each flag to the stage whose effect declares it —
 two effects must not declare the same param key.
 
+### Projects are plain JSON
+
+`src/core/project.cpp` reads and writes the whole setup — source paths, the
+chain with its params, canvas and seed — as a small JSON file, with its own
+minimal reader rather than a JSON dependency. Writing is driven by the effects'
+`ParamSpec`s, not by whatever happens to sit in a stage's `ParamValues`, so a
+key an effect does not declare is never written and never read back: an old
+project keeps the defaults for params that did not exist when it was saved.
+Source paths under the project's own folder are written relative, so a folder
+can be moved or shared whole.
+
+The interface opens and saves projects from the header-bar menu (New, Open,
+Save, Save As, with the usual Ctrl+N/O/S accelerators); the window title shows
+the project name. Opening one replaces the sources, so a decode still in flight
+is discarded rather than mixed into the new project (`dropLoadedImages_`).
+
 ### Two link targets, deliberately
 
 `effector_core` (static) holds image IO, params, effects, rng and links only