fix(orchestrator): read V3 ancestors as uncompressed instead of failing

[ValentaTomas]

Since #2919, createDiff/reloadSource handle an ancestor build missing from a V4+ header's Builds map by loading the ancestor's own header and requiring a self entry (SelfBuildData). Pre-V4 ancestor headers have no Builds map at all — appendAncestorBuilds deliberately skips them on upload — so the lookup fails, planRead errors, the guest gets EIO from NBD, and envd init returns 400. On staging this kills every resume of pre-V4-era templates: ~650 creates/h failing with INTERNAL since the ed6f90a deploy (previously 0), observable as the disappearance of the 1 GiB snapshot cohort and all pause-path rootfs uploads.

Fix: when the refreshed ancestor header is pre-V4, latch it as authoritatively uncompressed (UncompressedFullFrameTable, data at the basic path, size from storage) in both refresh paths — same semantics the read path had before #2919. V4+ headers keep the strict self-entry requirement (a missing self entry there still indicates a peer's in-flight header and must fail loudly).

Regression test: V4 header mapping to a V3 ancestor with no Builds entry now reads raw bytes from the uncompressed path.

[cursor]

PR Summary

Medium Risk
Changes core sandbox storage read paths for mixed V3/V4 ancestry; wrong branching could corrupt reads or mask in-flight peer headers, but scope is narrow and V4+ strict checks remain.

Overview
Fixes sandbox resume failures when a V4+ snapshot maps memfile pages to a pre-V4 (V3) ancestor that has no Builds map and no self entry in the current header.

After proactive header refresh, createDiff now treats ancestors with Metadata.Version < V4 as authoritatively uncompressed: open the basic data path with CompressionNone, return UncompressedFullFrameTable, and let size come from upstream (same as pre-#2919 V3 behavior). V4+ ancestors still require SelfBuildData and fail loudly on a missing self entry.

reloadSourceLocked gets the same pre-V4 branch so read-time refresh latches uncompressed upstream instead of erroring on self-entry lookup. Comments are updated to note V3 ancestors of V4 headers can hit this path.

Adds regression tests for V3-ancestor reads via createDiff and via direct StorageDiff reload.

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[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 6 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
packages/orchestrator/pkg/sandbox/build/build.go	50.00%	2 Missing and 1 partial ⚠️
...ges/orchestrator/pkg/sandbox/build/storage_diff.go	66.66%	2 Missing and 1 partial ⚠️

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[gemini-code-assist]

Code Review

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