build: single source of truth for the version

Version now lives only in [workspace.package] in the root Cargo.toml;
all three crates inherit it via version.workspace = true (this also fixes
gipfel-core, which had drifted to 0.1.0 while the others were 0.5.4).
edition/license are inherited the same way.

The About dialog now reads env!("CARGO_PKG_VERSION") instead of a
hardcoded string. A new test (changelog_top_matches_version) asserts the
top gipfel-X.Y.Z entry in ChangeLog equals the crate version, so the
manifest, the About dialog and the ChangeLog cannot drift apart.

AGENTS.md gains a "Releasing a new version" section documenting the flow
(bump workspace version, add ChangeLog entry, tag, git archive source
tarball).

7 files changed, 73 insertions, 13 deletions

diff --git a/AGENTS.md b/AGENTS.md
index f8992c7..29a4029 100644
--- a/AGENTS.md
+++ b/AGENTS.md
@@ -123,6 +123,35 @@ models. If you regenerate, re-run the golden tests immediately.
   `dirs` + exe-relative) and font fallback, then get libcosmic itself to compile
   for `x86_64-pc-windows-gnu` (upstream does not test Windows).
 
+## Releasing a new version
+
+The version has a **single source of truth**: `[workspace.package] version` in
+the root `Cargo.toml`. All three crates inherit it (`version.workspace = true`),
+and the app's **About dialog** reads it at compile time via
+`env!("CARGO_PKG_VERSION")`. Do **not** hardcode a version anywhere else.
+
+To cut a release `X.Y.Z`:
+
+1. Bump `version` in `[workspace.package]` (root `Cargo.toml`). That updates all
+   three crates and the About dialog at once.
+2. Add a new entry at the **top** of `ChangeLog`:
+   ```
+   gipfel-X.Y.Z
+   * one line per user-visible change
+   ```
+   The test `gipfel-app: changelog_top_matches_version` fails if the top
+   `gipfel-X.Y.Z` line does not equal the crate version, so the manifest and the
+   ChangeLog cannot drift apart.
+3. `cargo test --lib --test golden` (with `GIPFEL_DATA` set) — must be green.
+4. Commit, then tag: `git tag vX.Y.Z`.
+5. Build the **source** tarball (tracked files only, no build artifacts):
+   ```sh
+   git archive --format=tar.gz --prefix=gipfel-X.Y.Z/ \
+       -o ~/gipfel-X.Y.Z.tar.gz vX.Y.Z
+   ```
+   Recipients build with `cargo build --release`. For a fully offline tarball,
+   run `cargo vendor --versioned-dirs vendor/` and commit `vendor/` first.
+
 ## Do / Don't
 
 - ✅ Run `cargo test --lib --test golden` (with `GIPFEL_DATA` set) after any
diff --git a/Cargo.lock b/Cargo.lock
index 7518c3d..dca1d66 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1744,7 +1744,7 @@ dependencies = [
 
 [[package]]
 name = "gipfel-app"
-version = "0.1.0"
+version = "0.5.4"
 dependencies = [
  "cosmic-text",
  "fontdb",
@@ -1757,14 +1757,14 @@ dependencies = [
 
 [[package]]
 name = "gipfel-cli"
-version = "0.1.0"
+version = "0.5.4"
 dependencies = [
  "gipfel-core",
 ]
 
 [[package]]
 name = "gipfel-core"
-version = "0.1.0"
+version = "0.5.4"
 dependencies = [
  "dirs",
  "image",
diff --git a/Cargo.toml b/Cargo.toml
index 3a99088..d293395 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -2,6 +2,15 @@
 resolver = "2"
 members = ["crates/gipfel-core", "crates/gipfel-cli", "crates/gipfel-app"]
 
+# Single source of truth for metadata inherited by every crate via
+# `version.workspace = true` etc. Bump `version` here on release; the
+# About dialog and all crates follow. A test enforces the ChangeLog top
+# entry matches this version.
+[workspace.package]
+version = "0.5.4"
+edition = "2021"
+license = "GPL-2.0-or-later"
+
 [workspace.dependencies]
 kamadak-exif = "0.6"
 quick-xml = { version = "0.41", default-features = false }
diff --git a/crates/gipfel-app/Cargo.toml b/crates/gipfel-app/Cargo.toml
index 00cf451..f16f1c0 100644
--- a/crates/gipfel-app/Cargo.toml
+++ b/crates/gipfel-app/Cargo.toml
@@ -1,9 +1,9 @@
 [package]
 name = "gipfel-app"
-version = "0.5.4"
-edition = "2021"
+version.workspace = true
+edition.workspace = true
 description = "gipfel - photogrammetry for mountain images"
-license = "GPL-2.0-or-later"
+license.workspace = true
 
 [[bin]]
 name = "gipfel"
diff --git a/crates/gipfel-app/src/main.rs b/crates/gipfel-app/src/main.rs
index 967e68d..12e2845 100644
--- a/crates/gipfel-app/src/main.rs
+++ b/crates/gipfel-app/src/main.rs
@@ -1526,7 +1526,7 @@ impl Gipfel {
     fn view_about<'a>(&self) -> Element<'a, Message> {
         widget::dialog()
             .title("About gipfel")
-            .body("gipfel 0.5.4")
+            .body(format!("gipfel {}", env!("CARGO_PKG_VERSION")))
             .control(widget::text("Annotate mountain panoramas."))
             .control(widget::text("by Johannes Hofmann"))            .primary_action(widget::button::suggested("Close").on_press(Message::About(false)))
             .is_overlay(false)
@@ -1802,4 +1802,26 @@ mod tests {
         let (_, y_back) = pan.project_point(0.0, top).unwrap();
         assert!((y_back + 100.0).abs() < 0.01);
     }
+
+    /// Release drift guard: the top `gipfel-X.Y.Z` entry in the ChangeLog must
+    /// match the crate version (inherited from `[workspace.package]`). This
+    /// catches a bump of one but not the other. The About dialog reads the same
+    /// `CARGO_PKG_VERSION`, so all three stay in lockstep.
+    #[test]
+    fn changelog_top_matches_version() {
+        let path = concat!(env!("CARGO_MANIFEST_DIR"), "/../../ChangeLog");
+        let text = std::fs::read_to_string(path)
+            .expect("ChangeLog not found at the workspace root");
+        let top = text
+            .lines()
+            .map(str::trim)
+            .find_map(|l| l.strip_prefix("gipfel-"))
+            .expect("ChangeLog has no `gipfel-X.Y.Z` entry");
+        assert_eq!(
+            top,
+            env!("CARGO_PKG_VERSION"),
+            "ChangeLog top entry is `gipfel-{top}` but crate version is {}",
+            env!("CARGO_PKG_VERSION")
+        );
+    }
 }
diff --git a/crates/gipfel-cli/Cargo.toml b/crates/gipfel-cli/Cargo.toml
index 0a07982..b1369fa 100644
--- a/crates/gipfel-cli/Cargo.toml
+++ b/crates/gipfel-cli/Cargo.toml
@@ -1,9 +1,9 @@
 [package]
 name = "gipfel-cli"
-version = "0.5.4"
-edition = "2021"
+version.workspace = true
+edition.workspace = true
 description = "gipfel command line tools (export-position, export-hills)"
-license = "GPL-2.0-or-later"
+license.workspace = true
 
 [[bin]]
 name = "gipfel-cli"
diff --git a/crates/gipfel-core/Cargo.toml b/crates/gipfel-core/Cargo.toml
index d8ddfd9..6a66516 100644
--- a/crates/gipfel-core/Cargo.toml
+++ b/crates/gipfel-core/Cargo.toml
@@ -1,9 +1,9 @@
 [package]
 name = "gipfel-core"
-version = "0.1.0"
-edition = "2021"
+version.workspace = true
+edition.workspace = true
 description = "Port of gipfel's computational core (hills, panorama, projections, least-squares fit, metadata)"
-license = "GPL-2.0-or-later"
+license.workspace = true
 
 [dependencies]
 kamadak-exif.workspace = true