fix: expand sparse vehicle positions during streaming conversion

[fank]

Summary

• Fix static vehicles (DShKs, etc.) disappearing after a few seconds in streaming playback
• Fix incorrect endFrame calculation for vehicles using sparse position format
• Previously converted missions need re-conversion after this fix

Problem

Arma 3 vehicles can use a sparse position format where each position entry covers a range of frames via [startFrame, endFrame] at index 4. The JSON frontend handles this correctly via range-based lookup (_positionsHasFrames), but the backend parser ignored the frame ranges entirely — it assigned sequential frame numbers (startFrame + arrayIndex), treating all positions as dense.

For a static DShK with 1 sparse entry covering frames 0–5000, the parser created only 1 position (frame 0). The streaming writer then only included the entity in frame 0 of the chunk data, causing it to disappear after the first frame and reappear only when it started moving (generating more position entries).

This also caused calculateEndFrame to return startFrame + 1 instead of 5000, giving wrong entity lifetime metadata.

Fix

• collectEntityPositions: Detect sparse vehicle positions (entry[4] is [startFrame, endFrame] array) and expand each sparse entry into one EntityPosition per frame in the range
• calculateEndFrame: For sparse data, use the last entry's frame range end instead of startFrame + len(positions) - 1

Test plan

• TestParserV1_Parse_VehicleSparsePositions — verifies sparse→dense expansion (2 sparse entries → 15 dense positions)
• TestParserV1_Parse_VehicleSparsePositions_ChunkBuild — end-to-end: static vehicle appears in every frame of built chunk
• TestParserV1_Parse_VehicleDensePositions_Unaffected — regression: dense vehicle positions unchanged
• TestParserV1_calculateEndFrame_SparseVehicle — endFrame uses last sparse range end
• Full test suite passes

[github-actions]

Merging this branch will decrease overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/OCAP2/web/internal/storage	40.43% (-0.01%)	👎

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/OCAP2/web/internal/storage/parser_v1.go	39.28% (+0.05%)	1255 (+103)	493 (+41)	762 (+62)	👍

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/OCAP2/web/internal/storage/parser_test.go