Story 54: add cross-platform binary distribution support

- Add Makefile with build-macos and build-linux targets
  - build-macos: cargo build --release (native macOS binary)
  - build-linux: cross build --release --target x86_64-unknown-linux-musl
    (produces a fully static binary via Docker/cross; zero dynamic deps)
- Document cross-platform build process in README.md including
  how to verify macOS dynamic deps (otool -L) and Linux static
  linking (file + ldd)
- reqwest 0.13 already uses rustls by default (no OpenSSL); verified
  in Cargo.lock – no Cargo.toml changes needed
- Add unit tests to http/assets.rs covering:
  - SPA fallback routing for non-asset paths
  - 404 for missing assets/ paths
  - Panic-free behaviour on empty path
  - rust-embed EmbeddedAssets iter compiles and runs correctly

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

README.md

@@ -27,6 +27,55 @@ cargo build --release
 ./target/release/story-kit-server
 ```
 
+## Cross-Platform Distribution
+
+Story Kit ships as a **single self-contained binary** with the React frontend embedded via
+`rust-embed`. No Rust toolchain, Node.js, or extra libraries are required on the target machine.
+
+### macOS
+
+```bash
+# Native build – no extra tools required beyond Rust + pnpm
+make build-macos
+# Output: target/release/story-kit-server
+
+# Verify only system frameworks are linked (Security.framework, libSystem.B.dylib, etc.)
+otool -L target/release/story-kit-server
+```
+
+### Linux (static x86_64, zero dynamic deps)
+
+The Linux build uses the `x86_64-unknown-linux-musl` target to produce a fully static binary.
+
+**Prerequisites:**
+
+```bash
+# Install cross – a Rust cross-compilation tool backed by Docker
+cargo install cross
+
+# Ensure Docker Desktop (or Docker Engine) is running
+```
+
+**Build:**
+
+```bash
+make build-linux
+# Output: target/x86_64-unknown-linux-musl/release/story-kit-server
+
+# Verify the binary is statically linked
+file target/x86_64-unknown-linux-musl/release/story-kit-server
+# Expected: ELF 64-bit LSB executable, x86-64, statically linked
+
+ldd target/x86_64-unknown-linux-musl/release/story-kit-server
+# Expected: not a dynamic executable
+```
+
+**Running on any Linux x86_64 machine:**
+
+```bash
+# No Rust, Node, glibc, or any other library needed – just copy and run
+./story-kit-server
+```
 
 ## Testing