Query oracle and API freeze

Label matchers (equal, not-equal, regexp, not-regexp) in labels/,
postings intersection/union/difference in internal/postings/,
LabelValues and AllPostings on index, block, and head readers.

ingot.go wired to real DB backed by head + block readers. Querier
snapshots overlapping blocks, Select resolves matchers to postings,
merged iterator chains blocks then head with block-wins dedup.

Oracle tests compare every query against a naive []sample reference
across head-only, block-only, head+block merge seam, multiple blocks,
all four matcher types, and time range filtering.

internal/block/reader.go

@@ -105,6 +105,16 @@ func (r *Reader) Labels(ref uint64) ([]labels.Label, bool) {
 	return entry.Labels, true
 }
 
+// LabelValues returns sorted unique values for the given label name.
+func (r *Reader) LabelValues(name string) []string {
+	return r.idx.LabelValues(name)
+}
+
+// AllPostings returns sorted refs for all series in the block.
+func (r *Reader) AllPostings() []uint64 {
+	return r.idx.AllPostings()
+}
+
 // Close releases all resources (munmaps chunk files).
 func (r *Reader) Close() error {
 	return r.chunks.close()