Substance follows the same lazy_loaded_js pattern I approved in #33760 and commented on in #33778, and the body's claim that ext/io/ext/os/ext/net have no cross-extension ESM imports (only core.loadExtScript calls into already-converted extensions) checks out — no internals symbol bridges needed.

Real regression: IIFE wrapper shifts line numbers in stack traces

integration::run::permissions_trace fails on both debug macos-aarch64 and debug linux-aarch64 (so this is deterministic, not a single-platform flake). The expected output asserts:

┃  ├─ Object.hostname (ext:deno_os/30_os.js:43:10)

but actual output after the conversion is:

┃  ├─ Object.hostname (ext:deno_os/30_os.js:45:10)

Two-line shift — the IIFE wrapper ((function () { + setup) adds two lines at the top of 30_os.js, moving Object.hostname from line 43 to line 45. Stack-trace-pinning tests like permissions_trace see this directly.

The permissions_trace regression is the kind of thing #33760 / #33778 didn't trip on because their converted files (16_image_data.js, 00_crypto.js, etc.) weren't on any permissions_trace-style stack. ext/os/30_os.js's Object.hostname is — it's on the canonical "sys access requested" trace path.

Fix: update the expected output in tests/testdata/run/permissions_trace.out (the file the macros line tests/integration/run_tests.rs:430:1 points at) to the new line numbers. Or — cleaner long-term — switch the line numbers to [WILDLINE] so future IIFE/format changes don't cascade. The non-regenerated line-number assertion is going to keep biting on every script-conversion follow-up otherwise.

Other failure

test node_compat (2/3) debug windows-x86_64 fails on parallel/test-dns-resolver-max-timeout.js: timeout1: 505, timeout2: 515 — same Windows DNS-timing flake that's hit ~half the daily PRs. Pre-existing, unrelated.

Substance walk

• ext/io/lib.rs, ext/os/lib.rs, ext/net/lib.rs all switch the relevant esm = [...] entries to lazy_loaded_js = [...]. Right shape for the now-IIFE files.
• 23 files, +67/-47 net: most of the diff is the JS-side import → core.loadExtScript migration in consumers, plus the IIFE wrap on the converted files themselves. Smaller than #33778 because there's no equivalent of the giant 00_crypto.js patch.
• No internals shims — the body's statement that these extensions have no cross-ESM dependencies is verifiable from the diff: every JS-side import either stays import (for files still in ESM form) or migrates to core.loadExtScript (for already-converted extensions like ext/web).

CI

83/106 success at this head, 18 in flight, 3 failures (2 real permissions_trace, 1 unrelated DNS flake). Once the .out fixture is updated for the new line numbers, the substance side is clean and ready for re-CI.