Add story 50: cross-platform binary distribution with musl

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

.story_kit/stories/upcoming/50_cross_platform_binary_distribution.md

@@ -0,0 +1,26 @@
+---
+name: Cross-Platform Binary Distribution
+test_plan: pending
+---
+
+# Story 49: Cross-Platform Binary Distribution
+
+## User Story
+
+As a developer, I want to build self-contained binaries for macOS and Linux so that I can share Story Kit with others without requiring them to have a Rust toolchain.
+
+## Acceptance Criteria
+
+- [ ] `cargo build --release` produces a binary with no non-system dynamic dependencies on macOS (current state — verify)
+- [ ] CI or a documented process can produce a fully static Linux x86_64 binary using the `x86_64-unknown-linux-musl` target (via cross-compilation or Docker build)
+- [ ] The Linux binary has zero dynamic library dependencies (`ldd` reports "not a dynamic executable")
+- [ ] The frontend is embedded in the binary via `rust-embed` (current state — verify still works in release builds)
+- [ ] A Linux user can download and run the single binary without installing Rust, Node, glibc, or any extra libraries
+- [ ] Build instructions are documented in the project (e.g. a `Makefile` or `justfile` with `build-linux` / `build-macos` targets)
+
+## Out of Scope
+
+- Homebrew formula or package manager publishing
+- Windows support
+- Auto-update mechanism
+- Code signing or notarization