[Task]: [Go][Prism] Resolve TODO: Optimize watermark reads in stageState using min-heap

### What needs to happen?

In `beam/sdks/go/pkg/beam/runners/prism/internal/engine/elementmanager.go`, the function `minPendingTimestampLocked` performs a linear scan over `ss.pendingByKeys` every time the watermark refreshes.

As noted in the existing comment inside that function:

`// TODO(lostluck): Can we figure out how to avoid checking every key on every watermark refresh?`

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**The Fix:**
I will replace the current *O(K)* iteration over `ss.pendingByKeys` with a slice-based min-heap. This moves the retrieval of the minimum pending timestamp from a linear scan to a constant time (*O(1)*) lookup.

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**Technical Details & Impact:**
- **Scaling:** The read latency stays at a flat 13ns no matter the number of keys (tested from 1k to 1M keys) which replaces the current linear growth.

- **Memory Efficiency:** There is 0 allocation because it doesn't create a temporary memory while reading data which removes most actions from the garbage collector, keeping the scheduler efficient.

- **CPU Friendly:** Instead of going over the memory to look for a map, it grabs what it needs from an organized list reducing additional CPU computation.

- **Consistent Speed:** The read time is locked at around 13ns, no matter the size of the pipeline the system will not jitter or slow down.

### Issue Priority

Priority: 2 (default / most normal work should be filed as P2)

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