fix: Add abs() guard to Tymist.tock setter for consistency with Doer

[Nicholas-Keystate]

Summary

Add abs() guard to Tymist.tock setter for consistency with Doer.tock setter.

Problem

Tymist.tock.setter (tyming.py:78) did not apply abs():

self._tock = float(tock)  # Allowed negative!

But Doer.tock.setter (doing.py:569) does:

self._tock = abs(float(tock))  # Protected

Since Doist inherits from Tymist (not Doer), a Doist(tock=-X) would cause tyme to decrement on each tick(), breaking all scheduling invariants:

1. Tyme monotonicity violated
2. Tymers never expire (comparing against retreating tyme)
3. Periodic Doers' retymes never reached
4. Potential infinite loops

Fix

• Add abs() to Tymist.tock setter
• Update constructor to use setter (inherits abs() guard)
• Add test_tymist_tock_abs_guard test

Consistency

Other parameters already have sign guards:

• Doist.limit: abs(float(limit)) (doing.py:110)
• Doer.tock: abs(float(tock)) (doing.py:569)

This PR extends that pattern to Tymist.tock.

Test

tymist = tyming.Tymist()
tymist.tock = -1.0
assert tymist.tock == 1.0  # Now passes with this fix