bun: CI pins a single Bun version #23890

.github/workflows/build.yml:522 pins bun-version: '1.3.14' with no matrix, while Node runs a [20.19, 22, 24, 26] matrix. Bun ships breaking runtime changes often, and several of the findings above turn on Bun internals (Module.registerHooks presence, diagnostics-channel support, Bun.serve option shape).

Work item. Add a two-entry matrix: the pinned version plus latest, with continue-on-error on latest so it reports drift without blocking.

**Prior art **(related history only). #20890 (merged) is the PR that pinned Bun to 1.3.14. The drift it hides is already visible: #23521 (merged) "test(bun): Fix integration test on Bun 1.4.0" fixed a break found outside CI. No issue tracks adding a matrix.