feat: surface BuildKit root-cause on cancellation

[gilescope]

Depends on #572 (CI memory telemetry) — land this after #572 merges. It calls statsstreamparser.Parser.Reset(), introduced there. This branch bundles parser.go/parser_test.go so it builds standalone; once #572 is in main, those two files drop out as identical (no conflict).

Extracted from #442 (the buildkit upgrade) so the failure-visibility work can land independently of the bump.

What

Surfaces the real root cause of a failed build instead of a bare context canceled. When BuildKit cancels or loses the solve session after a vertex has already failed, earth now reports the original failing target/command and BuildKit error, and distinguishes client-side from daemon-side cancellation.

• logbus/solvermon/first_failure.go — captures the first fatal BuildKit vertex failure (scrubbed), preserved across the cancellation fan-out.
• logbus/solvermon/{solvermon,vertexmon}.go — record per-vertex failures + logs; reset the stats parser on a desynced stream.
• cmd/earthly/app/run.go — printCancellationOrigin reports whether cancellation began locally (signal / dead build context) or in BuildKit/the session layer; wires AsFirstFailureError into the fatal-error path.
• builder/solver.go — withBuildkitFailureContext attaches target/command context to the returned error.

Deliberately excluded (stays in #442)

The buildkit-API entitlements change in builder/solver.go/image_solver.go (AllowedEntitlements: s.enttlmnts → entitlementsToStrings(...) []string) — won't compile against current buildkit.

Verification

• go build ./... — green.
• go test ./logbus/solvermon/ ./builder/ — green.

[github-actions]

➖ Are we earthbuild yet?

No change in "earthly" occurrences

📈 Overall Progress

Branch	Total Count
main	5320
This PR	5320
Difference	+0

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Keep up the great work migrating from Earthly to Earthbuild! 🚀

💡 Tips for finding more occurrences

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[gemini-code-assist]

Code Review

This pull request enhances error reporting and cancellation handling in Earthly by tracking the first fatal vertex failure, first cancellation, and active operations during a build. It introduces detailed error types (FirstFailureError, FirstCancellationError, and CancellationDetailsError) and reports the origin of cancellations. Additionally, it improves the robustness of the stats stream parser by allowing it to recover from malformed frames instead of failing the build. The review feedback suggests avoiding if statements with initializers in builder/solver.go to comply with Go linting preferences.

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