fix(inbox): a segment title is declared, never marked derived

A segment's title comes from the plan, so a human adjudicated it. But
structure derivation runs over the segment body, finds no title key and
no usable heading, falls back to a stem, and adds "title" to derived.
The concept then emitted a stated fact under an inferred marker, and a
consumer that distrusts derived fields would distrust exactly the thing
a human decided. An over-marked field is the same defect class as an
unmarked heuristic: the marker is only worth something if it is
accurate in both directions.

Scoped to title alone, and pinned that way by test: number stays in
derived on a segment, because nothing about segmentation makes an
inferred document number declared. Without a segment a derived title is
still marked, so only a plan makes a title declared.

The SEGMENTED_V1 golden moves, which is the intended consequence and
the only golden that may. The four existing goldens are byte-identical
to baseline 770d8d4, measured against the sha rather than inspected.

Co-Authored-By: Claude <claude-opus-5>

examples/ingest-golden-segmented/expected-bundle/index.md

@@ -2,5 +2,5 @@
 bundle_id: b-golden-segmented
 ---
 
-- [Veiledning for eksempelbundel](veiledning.md) — derived: [title]
+- [Veiledning for eksempelbundel](veiledning.md)
 - [krav (index)](krav/index.md)

examples/ingest-golden-segmented/expected-bundle/krav/1-1/foerste-krav.md

@@ -6,7 +6,6 @@ source_sha256: 6906ec0acbcfc246e825bda9863c716eb5611b465020e8204eeb448c32343f7d
 ingested_at: 2026-08-30T09:00:00Z
 generated: true
 parent: s0
-derived: [title]
 bundle_id: b-golden-segmented
 segment_id: s1
 source_offset: [94, 176]

examples/ingest-golden-segmented/expected-bundle/krav/1-1/index.md

@@ -1 +1 @@
-- [Foerste krav](foerste-krav.md) — parent: s0?; derived: [title]
+- [Foerste krav](foerste-krav.md) — parent: s0?

examples/ingest-golden-segmented/expected-bundle/krav/1-2/andre-krav.md

@@ -6,7 +6,6 @@ source_sha256: 6906ec0acbcfc246e825bda9863c716eb5611b465020e8204eeb448c32343f7d
 ingested_at: 2026-08-30T09:00:00Z
 generated: true
 parent: s0
-derived: [title]
 bundle_id: b-golden-segmented
 segment_id: s2
 source_offset: [176, 253]

examples/ingest-golden-segmented/expected-bundle/krav/1-2/index.md

@@ -1 +1 @@
-- [Andre krav](andre-krav.md) — parent: s0?; derived: [title]
+- [Andre krav](andre-krav.md) — parent: s0?

examples/ingest-golden-segmented/expected-bundle/veiledning.md

@@ -5,7 +5,6 @@ source_file: veiledning.md
 source_sha256: 6906ec0acbcfc246e825bda9863c716eb5611b465020e8204eeb448c32343f7d
 ingested_at: 2026-08-30T09:00:00Z
 generated: true
-derived: [title]
 bundle_id: b-golden-segmented
 segment_id: s0
 source_offset: [0, 94]