feat(v2): add hello world grammar v2 example (#112)

Cherry-pick of `a99dc21` from #109 — the only commit on that branch
worth keeping after the v2 grammar work was re-landed on main via PRs
#46/#54/#57/#58/#62-#65/#69-#74/#76/#78/#80.

Adds `examples/v2/hello.eph` (13 lines) demonstrating the v2 grammar
surface: `module std/io`, `extern "wasm" { fn ... }`, `pub fn main`,
`let!` linear bindings.

Closes #109 (the rest was superseded — see close comment on #109).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: hyperpolymath

examples/v2/hello.eph

@@ -0,0 +1,13 @@
+module std/io
+
+extern "wasm" {
+    fn print_i32(val: I32): ()
+}
+
+pub fn main() -> I32 {
+    let! x = 40
+    let y = 2
+    let! result = x + y
+    print_i32(result)
+    result
+}