Add logging through slf4j (#6)

[xbt-a4224j]

Motivation:
Make requests, statuses and retries visible at a log level the app chooses, following the Python reference: same log points as transport.py:59-94, same redaction as SensitiveHeadersFilter, each log point one level below Python's so a stock Spring Boot app (root INFO) stays silent as a stock Python app (root WARNING) does.

Changes:

• Logging: slf4j logger io.github.premocloud.typesafe; wire() is the one redaction point, "***" for SECRET_HEADERS plus token/secret names
• TypeSafeClient: DEBUG line per request (status, elapsed, request id), per retry, per connection failure; TRACE for headers and bodies both ways
• build.gradle.kts: api(slf4j-api:2.0.16); logback-classic test-only
• LoggingTest: 5 tests on a captured appender, incl. key never logged and nothing emitted at INFO
• README Logging section, CHANGELOG entry

Result:
Stock Spring Boot (root INFO) sees no output. logging.level.io.github.premocloud.typesafe=DEBUG gives one line per request; TRACE adds the wire with Authorization=***. 44/44 tests pass.

Closes: #6

[xbt-a4224j]

@garretpremo Should we keep or remove the REQUESTS counter? It's in this PR to help distinguish logs of interleaved async calls, but maybe not worth.

[garretpremo]

@xbt-a4224j Adding an identifier to requests is super useful! Especially after adding async requests which could add interleaved logs that are indistinguishable from each other.

Suggested change: Generate the identifier as a short random hex id in the logging class rather than a JVM-global counter. This avoids collisions across restarts and replicas at a small cost to readability (which is worth it IMO for a debug line).

---

// Logging.java

/**
 * A short opaque tag for one logical call, shared by every attempt it makes, so the lines of
 * interleaved async calls can be told apart. Random rather than counted: no shared state, and no
 * collisions across restarts or replicas in an aggregated log.
 */
static String tag() {
    return String.format("req-%06x", ThreadLocalRandom.current().nextInt(1 << 24));
}

Call lines:

Suggested change

	new Call("req-" + REQUESTS.incrementAndGet(), requestBody));
	new Call(Logging.tag(), requestBody));

[garretpremo]

Pushed the random-id tag from the thread above to this branch (maintainer edit) along with a merge of main for the changelog, so this can go in with #9. Thanks for the PR!