feat(build): add --compile-executable-path CLI flag (#25934)

## Summary

Adds a new CLI flag `--compile-executable-path` that allows specifying a
custom Bun executable path for cross-compilation instead of downloading
from the npm registry.

## Usage

```bash
bun build --compile --target=bun-linux-x64 \
  --compile-executable-path=/path/to/bun-linux-x64 app.ts
```

## Motivation

The `executablePath` option was already available in the JavaScript
`Bun.build()` API. This exposes the same functionality from the CLI.

## Changes

- Added `--compile-executable-path <STR>` CLI parameter in
`src/cli/Arguments.zig`
- Added `compile_executable_path` field to `BundlerOptions` in
`src/cli.zig`
- Wired the option through to `StandaloneModuleGraph.toExecutable()` in
`src/cli/build_command.zig`

---------

Co-authored-by: autofix-ci[bot] <114827586+autofix-ci[bot]@users.noreply.github.com>

docs/runtime/ffi.mdx

@@ -358,7 +358,7 @@ Bun represents [pointers](<https://en.wikipedia.org/wiki/Pointer_(computer_progr
 
 **Why not `BigInt`?** `BigInt` is slower. JavaScript engines allocate a separate `BigInt` which means they can't fit into a regular JavaScript value. If you pass a `BigInt` to a function, it will be converted to a `number`
 
-**Windows Note**: The Windows API type HANDLE does not represent a virtual address, and using `ptr` for it will *not* work as expected. Use `u64` to safely represent HANDLE values.
+**Windows Note**: The Windows API type HANDLE does not represent a virtual address, and using `ptr` for it will _not_ work as expected. Use `u64` to safely represent HANDLE values.
 
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