tmpclient: add temporal decorrelation between context and identity requests

[bokelley]

The current tmpclient fires ContextMatch and IdentityMatch independently, but the two calls travel with nearly identical timing. A network observer correlating arrival times at the router and identity agent can re-link them even when they're sent in parallel.

Add an optional randomized delay (jitter) on the identity request to decorrelate the two calls. Suggested surface:

• WithDecorrelationMax(max time.Duration) Option — upper bound on the delay (default: off, recommend 5ms when enabled)
• Delay drawn from [0, max) via crypto/rand (not math/rand) before sending the identity request

Prior design lived in closed PR #9 (client/ package). That PR used math/rand/v2; we should use crypto/rand for this since it's privacy-adjacent.

Acceptance

• Option is off by default (no behavior change)
• When set, identity request is delayed by a random duration in [0, max)
• Test asserts delay occurs and is bounded
• No impact on context request timing

[bokelley]

Triage

Classification: Feature request
Bucket(s): tmpclient
Status: ready-for-human

What the experts said:

• code-reviewer: Two blockers — jitter sleep must be context-aware (select over ctx.Done(); time.Sleep ignores cancellation inside the goroutine) and no sleep-injection point makes the feature untestable without real wall-clock waits; both have clear prior art in webhook/publisher.go's contextSleep/fakeSleep pattern.
• dx-expert: Confirmed same two blockers; zero-means-disabled is idiomatic and correct; doc comment should explain the timing side-channel threat and note that WithDecorrelationMax adds to the effective latency budget.
• security-reviewer: Off-by-default is a footgun — every Activate caller is exposed today and opt-in means operators who need it most won't enable it; "Recommended: 5ms" overstates protection (5ms is indistinguishable from natural RTT noise against a passive observer at low publisher traffic rates, and meaningful window size depends on request rate).

My take: Conceptually sound and privacy-correct. Three design decisions needed before Execute is appropriate: context-aware sleep (fix is clear from webhook precedent), testable sleep injection (clear fix), and default-on vs off-by-default (requires @bokelley call on privacy posture). Flagging for human.

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Triaged by Claude Code. Session: https://claude.ai/code/session_01XoWGV6yd5RVzftebEbxZ18

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