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/index/reader.go

@@ -3,6 +3,7 @@ package index
 import (
 	"encoding/binary"
 	"hash/crc32"
+	"sort"
 
 	"git.dvdt.dev/david/ingot/labels"
 )
@@ -213,3 +214,29 @@ func (r *Reader) SeriesByRef(ref uint64) (SeriesEntry, bool) {
 func (r *Reader) Postings(name, value string) []uint64 {
 	return r.postings[labelPair{name, value}]
 }
+
+// LabelValues returns sorted unique values for the given label name.
+func (r *Reader) LabelValues(name string) []string {
+	seen := make(map[string]struct{})
+	for key := range r.postings {
+		if key.name == name {
+			seen[key.value] = struct{}{}
+		}
+	}
+	vals := make([]string, 0, len(seen))
+	for v := range seen {
+		vals = append(vals, v)
+	}
+	sort.Strings(vals)
+	return vals
+}
+
+// AllPostings returns sorted refs for all series in the index.
+func (r *Reader) AllPostings() []uint64 {
+	refs := make([]uint64, len(r.series))
+	for i, s := range r.series {
+		refs[i] = s.Ref
+	}
+	sort.Slice(refs, func(i, j int) bool { return refs[i] < refs[j] })
+	return refs
+}