[chagui]

Surfaces PatternExpanded events as a Slice[Map] attribute bazel.patterns on the root bazel.build span. Each entry is {pattern, target_count} aggregated by pattern across the invocation. Test-heavy repos that invoke bazel test //... get a self-describing trace — the difference between "asked for one thing" and "ran 1,400 tests" is now visible without correlating against external logs. Empty patterns and zero-target expansions are skipped.

Capped under high_cardinality_caps.patterns (default 50) following the #29 pattern. Truncation surfaces a warning log via the new []truncationRecord return from writeRootSpan — same shape as the BuildMetrics truncation pipeline. Aggregation is done over a map[string]int64 buffered on invocationState and stamped during writeRootSpan, identical to the workspace_items / build_metadata buffering. Empty buffer → no attribute, so absent vs present-but-empty stays a meaningful distinction.

One proto-shape note worth flagging: the pattern strings live on BuildEventId.pattern.pattern (repeated on the id), not on the PatternExpanded payload itself. The payload only carries test_suite_expansions. target_count is derived from len(event.children) — each child id is a target (or nested pattern) the expansion resolved to.

Required signature changes for the truncation plumbing: writeRootSpan now returns []truncationRecord, which threads through finalize and flushOrphaned. Both existing callers in tracebuilder.go updated to consume and warn-log the records. Distinct log message ("Truncated high-cardinality root-span attribute") so operators can filter from the BuildMetrics-side warning by namespace.

Closes #62.