feat(timing): add --timing request phase breakdown (#14)

[AzarAI-TOP]

Closes #14.

Adds a --timing flag that prints a per-phase breakdown of where a request spent its time, matching the issue's proposed output:

── TIMING ──────────────────────────────────────
  DNS Lookup         1ms
  TCP Connection     1ms
  TLS Handshake      0s
  Server Processing  704ms
  Content Transfer   0s
  Total              706ms

How it works

• Connect / TLS / request-written / first-response-byte are collected with net/http/httptrace.
• DNS is timed separately around the client's own concurrent resolver. kurl resolves hostnames in a custom DialContext (Cloudflare 1.1.1.1 raced against the system resolver), which httptrace's DNS hooks can't observe, so the --timing path records the DNS phase right there.
• The client returns once response headers are available — before the body is read — so it measures up to the first response byte and exposes TimeToFirstByte. main owns the wall-clock window around the body read and derives the content-transfer phase as Total - TimeToFirstByte.
• Phase durations use "latest wins" timestamps, so across a redirect chain a reused keep-alive connection correctly shows 0s for the phases it skipped rather than stale values.
• --timing is persisted by kurl save / kurl run.

Acceptance criteria

• Add --timing flag
• Measure and display DNS, TCP, TLS, TTFB (server processing), transfer phases
• Use Go's httptrace package for accurate measurements (plus explicit DNS timing for kurl's custom resolver)
• Format output cleanly aligned
• Add test coverage

Tests

• client: phase() helper edge cases; Fetch populates timing against a local httptest server (TCP/TTFB positive, TLS zero for http) and leaves Timing nil when not requested.
• main: --timing flag parsing (set and default).
• printer: rendered output format includes every phase label and value plus Total.

go test ./..., go vet ./..., and gofmt are all clean. Verified manually against both HTTPS (Cloudflare) and plain-HTTP endpoints.

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