add concurrent-code seams and hermetic tests for statsd, WaitLoop, Monitor

[dolph]

Summary

go test -race was running in CI, but no test exercised the package's concurrent surfaces, so the race detector had nothing to observe — a false signal. This PR adds the minimum testability seams to drive each concurrent path from a hermetic test, plus four tests that actually run that code under -race.

Seams added (existing public signatures unchanged; new helpers are unexported lowercase variants):

• statsd.go — count / timer / gauge / increment / statsdSender now take a chan<- string (or <-chan string) parameter. Public Count / Timer / Gauge / Increment / StatsdSender route through the helpers with the package-level queue as the default. Tests inject their own buffered channel.
• destinations.go — Monitor's loop body is factored into monitorWithCheck(confidence, check, sleep) int (one iteration, returns the next confidence). WaitFor's loop is factored into waitForWithCheck(check, sleep). The infinite for {} in Monitor stays in the public method.
• connectivity.go — waitLoop(destinations, checks, sleep) is the testable form of WaitLoop, letting tests drive the goroutine fan-out with stub checks and a zero sleep.

Tests added (concurrency_test.go):

• TestStatsdCountBlocksWhenQueueFull — pins the Statsd emitter can stall checks when statsd is down; opens a new connection per metric #11 back-pressure hazard by saturating an injected queue and asserting the next count send blocks. Marked Refs #11 -- flip when fixed so a future non-blocking-send PR knows to invert the assertion.
• TestStatsdSenderDeliversToUDP — stands up a real UDP listener on 127.0.0.1:0, runs the sender goroutine, fans 4 producers x 5 metrics through the injected queue, and asserts all 20 datagrams arrive with the right wire format. Real producer/consumer execution under -race.
• TestWaitLoopCompletesWhenChecksSucceed — drives the goroutine fan-out with stub checks that succeed on the second attempt; asserts every destination's check was invoked at least twice and that runtime.NumGoroutine() returns to the baseline (catches a leaked waitForWithCheck goroutine).
• TestMonitorWithCheckResetsConfidenceOnFailure — pins the post-monitor cadence delays outage detection by tens of minutes after a healthy period #16 confidence reset: after 12 successes saturate confidence at 10 (sleep = 10m), one failure snaps the next sleep back to 1m.

Scope

This is testability-only — no user-visible behavior changes:

• Does not fix Statsd emitter can stall checks when statsd is down; opens a new connection per metric #11 (producers still block on a full queue; that's the hazard the new test pins).
• Does not fix Process lifecycle is brittle: no signal handling, no statsd drain, no panic recovery #17 (no signal handling, no statsd drain, no panic recovery, no context.Context). // see #17 doc comments mark the spots where that work needs to land.
• Does not fix wait has no overall timeout — orchestrators hang indefinitely on permanently-broken dependencies #18 (no --timeout flag, no exponential backoff in WaitFor). // see #18 doc comment marks the 15 * time.Second sleep.

Per AGENTS.md TDD

Each new test was verified by mutation testing: temporarily breaking the production code (making count non-blocking; dropping bytes in statsdSender; skipping the retry in waitForWithCheck; dropping the confidence reset in monitorWithCheck) caused the matching assertion to fail, confirming the test actually exercises the failure mode rather than passing trivially.

Test Plan

• go vet ./...
• gofmt -l . (no output)
• go build ./...
• go test -race ./... — 4 new tests pass; pre-existing TestRouteToLoopback{1,2,3} failures from sandbox routing (Tests aren't hermetic and major modules are untested; production code is hard to test #24) remain on origin/main and are not introduced by this PR.
• go test -race -count=10 -run "TestStatsd|TestWaitLoop|TestMonitor" — no flakiness.
• Mutation: each test fails when the production code path it claims to cover is broken.

Fixes #49

https://claude.ai/code/session_01WjHPSobuzrRkjwUgjAJWMk

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