engine: explain why each candidate build was skipped in Flutter web loader (#186254)

## Description

When the Flutter web loader picks a build for the current browser, it
walks `buildConfig.builds` and takes the first one whose
`compileTarget`, `renderer`, and WASM-allowlist constraints all match.
Today, builds that fail those checks are silently filtered out, so:

* If every build is rejected, the user sees the generic *"FlutterLoader
could not find a build compatible..."* error and has no way to tell
which constraint blocked which build.
* If a preferred build (e.g. `dart2wasm` + `skwasm`) is rejected and the
loader silently falls back to a less-preferred build (e.g. `dart2js` +
`canvaskit`), the user has no way to tell that a fallback even happened,
much less why.

This was suggested by @yjbanov in
https://github.com/flutter/flutter/issues/143603#issuecomment-1996739832
and is the fallback-side complement to the COOP/COEP load-failure
warning in #142822.

## What changed

`loader.js`'s `buildIsCompatible` was a `bool`-returning predicate.
Refactored to `buildIncompatibilityReason` that returns either `null`
(compatible) or a short human-readable string. The selection loop
captures the reason for each skipped candidate, then:

* If **no compatible build is found**, prints a `console.warn`
unconditionally so the silently-blank-page case is never silent. The
warning hints at `verboseBuildSelection` for follow-up.
* If **`verboseBuildSelection: true`** is set on the Flutter config,
prints one `console.warn` per skipped candidate explaining its rejection
reason — useful both for the no-build case and for the "why does the
loader keep falling back to canvaskit when I expected skwasm?" debug
case. Off by default to keep the typical user's console quiet.

Sample output a developer would see on a browser without WasmGC, with
`verboseBuildSelection: true`:

```
[warn] Flutter Web: build dart2wasm/skwasm was skipped: dart2wasm requires WasmGC support; this browser does not implement it yet.
```

And without `verboseBuildSelection`, when no compatible build is found:

```
[warn] Flutter Web: no compatible build found for this browser. Set `verboseBuildSelection: true` in your Flutter configuration to see why each candidate was rejected.
```

## Tests

No JS test infrastructure exists for
`engine/src/flutter/lib/web_ui/flutter_js/`. The change is also pure
additive logging — the build-selection algorithm picks the same build it
picked before. Requesting a test exemption.

This PR replaces #186238 (closed for a clean conversation timeline).

## Pre-launch Checklist

- [x] I read the [Contributor Guide].
- [x] I read the [Tree Hygiene] page.
- [x] I read and followed the [Flutter Style Guide].
- [x] I signed the [CLA].
- [x] No behavioral change beyond the new opt-in diagnostic logging.

[Contributor Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md#overview
[Tree Hygiene]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Tree-hygiene.md
[Flutter Style Guide]:
https://github.com/flutter/flutter/blob/master/docs/contributing/Style-guide-for-Flutter-repo.md

---------

Co-authored-by: Harry Terkelsen <1961493+harryterkelsen@users.noreply.github.com>