chore: initial scaffold for latprobe CLI tool

Sets up project structure, working conventions (CLAUDE.md), README with
the full assignment and roadmap, seeded CHANGELOG, and a buildable Go
scaffold (Step 0) with flag parsing and stub probe types.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

go/internal/probe/probe.go

@@ -0,0 +1,34 @@
+// Package probe measures per-phase HTTP request latency.
+package probe
+
+import "time"
+
+// Phase holds the measured duration of a single request phase.
+// A zero Duration means the phase did not occur (e.g. TLS on plain HTTP).
+type Phase struct {
+	Duration time.Duration
+	// Present is false when the phase was skipped (e.g. no TLS for http://).
+	Present bool
+}
+
+// Result holds all timing phases for a single HTTP request.
+type Result struct {
+	URL      string
+	DNS      Phase
+	Connect  Phase
+	TLS      Phase
+	TTFB     Phase // server processing: WroteRequest → GotFirstResponseByte
+	Transfer Phase // body read: GotFirstResponseByte → body closed
+	Total    Phase
+
+	// StatusCode is the HTTP response status code (0 on error).
+	StatusCode int
+	// Err is non-nil if the request failed.
+	Err error
+}
+
+// Measure performs an HTTP GET request to url and returns a populated Result.
+// It is a placeholder until Step 1 wires in the actual implementation.
+func Measure(url string) Result {
+	return Result{URL: url}
+}