Summary

Fix update cache invalidation across sbt command invocations when a dependency's dependencies change.

Problem

When project A's dependencies changed and A was compiled in one command, project B (depending on A) would not invalidate its update cache in a subsequent command. This caused stale classpaths.

The root cause was that depsUpdated only checked !stats.cached, which only detected fresh resolves within the same command. When a dependency was served from cache (even if resolved fresh in a previous command), cached was marked true, causing incorrect cache reuse.

Debug scenario from issue:
sbt:aaa> clean
sbt:aaa> a/compile
sbt:aaa> show itTests/depsUpdated
[info] * false <-- BUG: should be true

Solution

Added stamp: String field to UpdateStats that records when the update was resolved. This stamp persists across commands and enables accurate cross-command comparison:

• If any dependency's stamp > our cached stamp, we re-resolve
• Falls back to !cached check for backwards compatibility with old caches
• Using String type allows future transition from timestamps to content hashes

Changes

• Add stamp: String field to UpdateStats schema (default "" for compat)
• Set stamp = System.currentTimeMillis().toString when creating UpdateStats
• Modify cachedUpdate to compare stamps instead of just cached flag
• Add backwards-compatible JSON deserialization for old cache files
• Add scripted test i8357-transitive-update-invalidation

Test plan

• Compiles successfully
• lmCore/test passes
• Run scripted test: scripted dependency-management/i8357-transitive-update-invalidation

Fixes #8357

Contribution by Gittensor, see my contribution statistics at https://gittensor.io/miners/details?githubId=148254234