fix: commit response status before flushing

[Plastic-time]

Problem

Flushing an Echo response before writing its body commits the underlying HTTP headers, but leaves Echo's Committed flag false and skips Before callbacks. A staged status such as 202 also becomes 200 on the wire. The delayed status writer has the same discrepancy.

Commit the configured status (defaulting to 200) before delegating Flush in both writers. This makes subsequent writes observe the already committed response and runs Before hooks exactly once.

Validation

Go 1.27.3 on Windows:

• Four regression subtests failed before the implementation change (default/custom statuses for both writers).
• go test -race ./... passed, including middleware and echotest.
• go vet ./... passed.
• go run honnef.co/go/tools/cmd/staticcheck@latest ./... passed.
• go run golang.org/x/lint/golint@latest -set_exit_status ./... passed.
• gofmt and git diff --check passed.

Tests also cover headers added by Before hooks, repeated Flush calls, and a body write after flushing. No new public API or dependencies.

AI assistance

Codex (GPT-6) assisted with investigation, implementation, tests, and this description. Submitted as a draft for human review.