perf: cache SharedString table as O(1) List on first access

[IoannisMaras]

Every cell read and rich-text span format calls SharedStringTable.Elements<SharedStringItem>().ElementAtOrDefault(idx). In the OpenXML SDK, SharedStringTable is a linked list. Walking from node 0 to `idx` on every cell performs millions of redundant pointer hops.

Fix

1. Cache SharedStringTable in a random-access `List` on first access (`GetSharedStringItemCache()`).
2. Update `GetCellDisplayValue` and `TryBuildRichTextHtml` to use the O(1) cache.
3. Call `InvalidateSharedStringCache()` whenever new entries are appended (`add cell`), preserving 100% write correctness.

5-Run Benchmark Results

• `view text`: 1369ms -> 933ms (~436ms saved, 1.47x speedup)
• `view html`: 1851ms -> 1090ms (~761ms saved, 1.70x speedup)
• `view screenshot`: 3078ms -> 2372ms (~706ms saved, 1.30x speedup)"