fix: effect teardown on failed setup, computed throw recovery

[nickchomey]

This PR fixes a few related bugs. It comes in 3 pairs of red tests/green fix commits.

Note that this Supersedes/rebases #119, which fixes #118. I didnt create an issue for the two new bugs i found and fixed

Bug 1 - effect / effectScope leave live subscriptions after failed setup

Tests:

• Stopped effect stays stopped. Reads signal, stops mid-run, reads another signal. Ends with no deps, never re-runs.
• Failed effect setup cleaned up. Reads signal then throws. Throw propagates. Later writes do not re-run.
• Failed scope setup cleans up children. Creates child effect then throws. Throw propagates. Later writes do not re-run child.

Fix:

• Failed setup leaves nothing behind.
• Inactive subscribers ignore later reads in same run.

Bug 2 - computed throwing on first evaluation never recovers

Tests:

• Throwing computed recovers. Throws while dependency is 0, read inside catching effect. After dependency becomes 5, effect re-runs and reads 10.

Fix:

• Failed evaluation still subscribes.
• Dependency change triggers re-evaluation.

Bug 3 - throwing computed returns stale undefined instead of re-throwing

Tests:

• Unrelated re-run still throws. Subscriber also reads unrelated signal. Write to unrelated signal re-runs subscriber without changing computed deps. Same error observed, getter not re-invoked.
• Direct re-read still throws. Two consecutive reads throw with single getter call. After dep change, correct value returned.

Fix:

• Failed evaluation is retained.
• Unchanged dependencies re-throw, changed dependencies re-evaluate.

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