feat!: axum 0.8, plus the extractors and router methods it was hiding

[Kilerd]

Three things at once, because they belong in the same breaking release.

1. axum 0.7 → 0.8

The headline change is path syntax: a captured segment is written {id}, not :id, and axum refuses the old form at startup rather than matching it literally:

Path segments must not start with `:`. For capture groups, use `{capture}`.

That is a breaking change for every application's routes, which is exactly why it belongs here rather than in 0.5 — asking users to migrate their config in 0.4 and their routes in 0.5 is two disruptions where one will do.

It also simplifies gotcha: {id} is what OpenAPI already used, so the framework no longer translates between the two syntaxes. replace_path_variable becomes the identity (kept, documented, and tested as such), and the parameter-name extraction in ParameterProvider reads {name}.

Other adjustments the upgrade forced:

• axum-core 0.5 dropped #[async_trait] from its extractor traits, so Valid, Header and Cookie use a plain async fn — reversing the attribute that E0195 demanded under 0.7.
• Router::layer gained Sync bounds, mirrored on GotchaRouter::layer and the builder.
• axum-extra 0.10, axum-macros 0.5, axum-prometheus 0.8; the two examples carrying their own axum dependency move too.
• http = "0.2" dropped — unused, and two major versions behind what axum itself uses.

2. fallback_service

Unmatched requests can now go to a Service rather than a handler — serving an SPA's index.html with ServeFile, or forwarding to a proxy, where fallback would need a wrapper.

3. The extractors that were there but unreachable

Re-exported, so none of these need gotcha::axum::… any more:

	note
Form, Multipart	the features were already enabled and Multipart already had a ParameterProvider impl — they were simply never re-exported
Sse, Event, KeepAlive	needed no new axum feature
WebSocketUpgrade, WebSocket	turns on axum's ws feature
middleware	for middleware::from_fn
MatchedPath, OriginalUri	turns on matched-path / original-uri

The websocket frame type stays behind ws::Message, because Message is already the message-system trait — re-exporting it bare would have been an ambiguity.

Tests

tests/pass/handler/axum_surface.rs exercises all of it: a urlencoded body, a multipart upload, an SSE stream, a websocket upgrade that echoes a frame, MatchedPath/OriginalUri, middleware::from_fn, and fallback_service.

Migration

MIGRATION.md now leads the 0.4 section with the route-path change, since it is the edit every application must make — ahead of the configuration change it previously led with.

Verification

Workspace builds with --all-features, every feature combination tests, clippy --all-features --workspace, fmt.

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