chore: extract shared render, watcher, and HMR modules

[janechu]

Summary

• Moved render.rs, watcher.rs, and HMR version logic into examples/shared/rust/ so both the tiny_http and hyper integration examples reference a single copy via #[path] includes
• Extracted hmr_version() as a shared pure function in examples/shared/rust/hmr.rs, replacing duplicated inline logic in each example's routes/hmr.rs
• Deleted the local render.rs and watcher.rs from both integration examples (net removal of ~350 lines of duplicated code)

Test plan

• cargo build succeeds for both tiny_http and hyper examples
• cargo clippy -- -D warnings passes for both examples
• Manual test: cd examples/integration/tiny_http && cargo run -- --app hello-world serves correctly
• Manual test: cd examples/integration/hyper && cargo run -- --app hello-world serves correctly
• HMR polling (/hmr endpoint) returns version timestamps in both examples

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