fix: allow zero close for untouched channels

[bpierre]

Summary

• allow close(0) when a session channel is still untouched (spent = 0, settled = 0)
• keep the existing strict > settled rule once settlement has advanced
• add a regression test covering zero-close on an untouched channel

Context

An untouched channel should be able to cooperatively close at 0 instead of falling back to a forced close grace period.

Testing

• add a regression test in src/tempo/server/Session.test.ts