fix(cli): format page bundler failures

[oxura]

Fixes #1399.

Move the HTML dispatch under the existing CLI error boundary and await it there. Preserve AggregateError child diagnostics instead of replacing the actual missing-page/output-path reason with AggregateError: Bundle failed. CLI option ordering and successful bundling are unchanged. Document nonzero page-build failure behavior in the guide; permanent token caps are unchanged.

Verification on Linux x86_64 / Bun 1.3.13:

• Before: missing HTML and an existing-file output path exit 1 with Bun's unhandled-error banner.
• After: both exit 1 with Error: Bundle failed plus their concrete BuildMessage; no unhandled-error banner. The existing output file remains byte-identical.
• A valid static HTML page still builds and exits 0.
• Invalid HTML options and a missing .bend entry retain their existing errors.
• bun gates/repo.ts: PASS: 50 / 50.

No browser behavior change is claimed. The separately reported empty-chunk/import-validation issue #1400 is not addressed. No mini-cluster gates were run; this is a local actual-CLI before/after smoke, not a mocked Bun.build test.

[nicolas-abril]

Thanks, @oxura! Moving the page build under the CLI's error handling was the right fix. Before merging I pushed one commit on top (ab6b689):

• Errors keep their location. The AggregateError branch formatted each bundler error with String(), which dropped its file, line, column and code frame. For a page whose import doesn't resolve, that left only Error: Bundle failed and ResolveMessage: Could not resolve: "./nothere.ts", with no hint of which module. For a failing .bend import it lost the at …/bad.bend:0 line, so you couldn't tell which .bend file failed. book_err now prints each one with Bun.inspect(m, { colors: false }), which keeps the frame and at file:line:col. Each line starts with the level (error:) rather than Bun's class name, and there's no separate Error: Bundle failed header.
• No blank line on an empty list. An AggregateError with no child errors prints just itself.
• A page rejects flags it would ignore. bend page.html -o dist --verdict and bend page.html extra -o dist used to build and exit 0. They now stop with "a page bundles with -o ", the way a .bend file rejects options that don't apply to it.
• Smaller diff. The option checks stay where they were on main. Only the cli_bundle call moves inside the try, which is all it needs to go through the CLI's error output.
• Guide reverted. We're keeping the guide as it was.
• Regression check. tests/ only holds .bend programs compared against their #| output, so this lives in gates/ping.ts, which already runs the installed bend executable. It builds a page with an unresolvable import and expects exit 1, the Could not resolve message, page.ts:1:, and no Bun v crash banner. It fails on the earlier version of this PR and passes now.

Checked with the real CLI: a missing import, a failing .bend import, a missing page, -o pointing at an existing file (left untouched), --verdict and an extra argument all exit 1 with no crash banner, and a valid page still builds. gates/_run.ts gives repo 50/50, ping 47/47 and test 1587/1587.

[oxura]

Thanks for the follow-up and merge. Preserving each bundler diagnostic’s code frame/location and rejecting arguments a page would ignore is the right behavior. gates/ping.ts is the appropriate home for the installed-CLI regression. I will preserve your ab6b6897 follow-up and the reverted guide, and use that diagnostic/test-placement convention for future CLI fixes.