ADR: Channel size estimation is not supported

[endrju19]

Summary

• Adds ADR documenting the decision to not expose Channel.estimateSize() or any size estimation on channels
• The lock-free segment-based algorithm tracks send/receive attempts (not completions), so interrupted operations permanently inflate any naive senders - receivers estimate
• Kotlin's kotlinx.coroutines (same algorithm) deliberately omits any size method for the same reason
• Users who need channel-level metrics should build external solutions (e.g., wrapping send/receive with their own counters)

Resolves #189. Supersedes #254.