lib-vt: wasm convenience functions and a simple example (#9309)

This adds a set of Wasm convenience functions to ease memory management.
These are all prefixed with `ghostty_wasm` and are documented as part of
the standard Doxygen docs.

I also added a very simple single-page HTML example that demonstrates
how to use the Wasm module for key encoding.

This also adds a bunch of safety checks to the C API to verify that
valid values are actually passed to the function. This is an easy to hit
bug.

**AI disclosure:** The example is AI-written with Amp. I read through
all the code and understand it but I can't claim there isn't a better
way, I'm far from a JS expert. It is simple and works currently though.
Happy to see improvements if anyone wants to contribute.

example/wasm-key-encode/README.md

@@ -0,0 +1,76 @@
+# WebAssembly Key Encoder Example
+
+This example demonstrates how to use the Ghostty VT library from WebAssembly to encode key events into terminal escape sequences.
+
+## What It Does
+
+The example demonstrates using the Ghostty VT library from WebAssembly to encode key events:
+
+1. Loads the `ghostty-vt.wasm` module
+2. Creates a key encoder with Kitty keyboard protocol support
+3. Creates a key event for left ctrl release
+4. Queries the required buffer size (optional)
+5. Encodes the event into a terminal escape sequence
+6. Displays the result in both hexadecimal and string format
+
+## Building
+
+First, build the WebAssembly module:
+
+```bash
+zig build lib-vt -Dtarget=wasm32-freestanding -Doptimize=ReleaseSmall
+```
+
+This will create `zig-out/bin/ghostty-vt.wasm`.
+
+## Running
+
+**Important:** You must serve this via HTTP, not open it as a file directly. Browsers block loading WASM files from `file://` URLs.
+
+From the **root of the ghostty repository**, serve with a local HTTP server:
+
+```bash
+# Using Python (recommended)
+python3 -m http.server 8000
+
+# Or using Node.js
+npx serve .
+
+# Or using PHP
+php -S localhost:8000
+```
+
+Then open your browser to:
+
+```
+http://localhost:8000/example/wasm-key-encode/
+```
+
+Click "Run Example" to see the key encoding in action.
+
+## Expected Output
+
+```
+Encoding event: left ctrl release with all Kitty flags enabled
+Required buffer size: 12 bytes
+Encoded 12 bytes
+Hex: 1b 5b 35 37 3a 33 3b 32 3a 33 75
+String: \x1b[57:3;2:3u
+```
+
+## Notes
+
+- The example uses the convenience allocator functions exported by the wasm module
+- Error handling is included to demonstrate proper usage patterns
+- The encoded sequence `\x1b[57:3;2:3u` is a Kitty keyboard protocol sequence for left ctrl release with all features enabled
+- The `env.log` function must be provided by the host environment for logging support
+
+## Current Limitations
+
+The current C API is verbose when called from WebAssembly because:
+
+- Functions use output pointers requiring manual memory allocation in JavaScript
+- Options must be set via pointers to values
+- Buffer sizes require pointer parameters
+
+See `WASM_API_PLAN.md` for proposed improvements to make the API more wasm-friendly.