TMP client library for publishers

[bokelley]

Summary

• Publisher-facing client that handles the full TMP match flow
• Fires context and identity requests in parallel with temporal decorrelation (configurable random delay on identity request to prevent timing correlation)
• Joins results locally: package eligible only if context offers it AND identity approves
• Expose endpoint for post-impression frequency cap updates
• Functional options: timeout, decorrelation max, HTTP client, retry with exponential backoff
• Structured error types wrapping TMP ErrorResponse

Test plan

• Parallel execution verified (both endpoints called)
• Temporal decorrelation verified (identity request delayed)
• Join logic: no context offer = not eligible, identity says no = not eligible
• Signals pass-through from context response
• Error handling: context failure, identity failure, timeout, HTTP errors
• Expose flow
• All tests pass with -race

Closes #7

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