Simplify build mapping resolution

[ValentaTomas]

Remove the bitset and simplify build id lookup to reduce the memory usage.

[cursor]

PR Summary

Medium Risk
Mapping resolution now relies on Header.Mapping being sorted/contiguous and uses binary search; if callers ever provide unsorted or gapped mappings, lookups may return the wrong source or fail. Behavior changes could surface as read offset/mapping errors in storage paths that call GetShiftedMapping.

Overview
Simplifies Header build mapping resolution by removing the precomputed bitset/start-index map and switching getMapping to a sort.Search over Header.Mapping, computing shifts directly from the chosen range. This also trims related block-based error/log details and removes the extra allocation/indexing work done during NewHeader construction.

^{Reviewed by Cursor Bugbot for commit b90faf6. Bugbot is set up for automated code reviews on this repo. Configure here.}

[cursor]

Cursor Bugbot has reviewed your changes and found 1 potential issue.

Bugbot Autofix prepared a fix for the issue found in the latest run.

• ✅ Fixed: Binary search assumes sorted mapping without enforcement
  • Added sort.Slice in NewHeader to guarantee mappings are sorted by Offset, ensuring binary search correctness regardless of input order.

Or push these changes by commenting:

@cursor push 277fabe9bc

Preview (277fabe9bc)

diff --git a/packages/shared/pkg/storage/header/header.go b/packages/shared/pkg/storage/header/header.go
--- a/packages/shared/pkg/storage/header/header.go
+++ b/packages/shared/pkg/storage/header/header.go
@@ -32,6 +32,10 @@
 		}}
 	}
 
+	sort.Slice(mapping, func(i, j int) bool {
+		return mapping[i].Offset < mapping[j].Offset
+	})
+
 	return &Header{
 		Metadata: metadata,
 		Mapping:  mapping,

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