chore(lr-5e24): static ESM import-resolution check for lib/public/

[akuehner]

Summary

• Adds test/esm-import-check.test.js — a static analysis pass that walks every .js file in lib/public/, extracts named imports from relative specifiers, and verifies each imported name is actually exported by the target module.
• Catches the failure class from lr-8657 / PR fix(lr-8657): repoint stale pending-navigate imports — fixes console stuck on 'Reconnecting' #223: stale named imports pointing to a file where the export has moved or been renamed. All 276 existing tests pass; the new check is test #277, running in under 2ms with zero new dependencies.

What is checked

• Every relative import path resolves to an existing file.
• Every named symbol in import { a, b } actually appears as an export in the target file.

What is intentionally skipped

• Bare specifiers (node:test, ws, etc.) — no local file to check.
• Default imports — no stable name to verify.
• export * from chains — accepted without deep-following to avoid false-positives on unbounded re-export graphs.

Test plan

• npm test: 277/277 pass, 0 failures
• New test runs in under 2ms (measured: 1.84ms)
• No new npm dependencies

Regression proof

A stale import { getPendingNavigate } from ./filebrowser.js (after the export moved to pending-navigate.js) would produce:
MISSING EXPORT app.js imports { getPendingNavigate } from ./modules/filebrowser.js — not exported by modules/filebrowser.js

Task: lr-5e24