feat(retrieval): add bounded ranked-prefix materialization

[ohdearquant]

Stack and governance gate

• Stacked on feat(storage): implement attachment cutover (ADR-160 Phase 4b) #2014 (codex/adr-160-phase-4-attachment-cutover).
• Implements ADR-160 Phase 5 only.
• ADR-160 was accepted on main in 135c3d46 (docs(adr): mark ADR-160 accepted #2005). Keep this PR draft until the prerequisite Phase 4a/4b operational rollout gates are satisfied and Claude completes review.

What changed

• Add a root-exported, runtime-independent khive-retrieval ranked-prefix materialization controller with bounded inputs, batches, drop taxonomy, counters, and diagnostics.
• Preflight limits, taxonomy, uniqueness, and strict ordering before any callback or loader I/O.
• Validate each returned loader batch structurally before classification, preserve caller errors, stop classification at K, and validate the remaining tail without loader/classifier I/O.
• Replace moodboard's pack-local namespace merge with khive_fusion::union_fusion and migrate candidate materialization to the shared controller with loader_batch_size = 1.
• Preserve moodboard's existing score/self/entity/scope/kind/subtype/content-ref/blob-existence precedence, one-shot overfetch, compact ranks, and honest underfill.
• Document the opaque caller-owned score boundary and publish-order dependency.

Regression guards

• Controller tests pin exact ceilings, invalid taxonomies, duplicate/non-monotonic input, malformed loader rows and deterministic error precedence, non-Clone scores, zero-output behavior, K/tail behavior, and bounded diagnostics.
• Moodboard tests pin canonical union ties and global truncation, self-hit zero I/O, post-K validation-only behavior, earlier loader-error precedence, typed drop accounting, honest underfill, soft-deleted entities, and malformed compatibility-projected refs.
• TDD RED was an intentional compile failure on the missing shared/moodboard seams; GREEN is the implementation in this PR.

Validation

• RUSTC_WRAPPER= cargo check --workspace -j4
• RUSTC_WRAPPER= cargo clippy --workspace --all-targets -- -D warnings
• RUSTC_WRAPPER= cargo test --workspace -j4
• cargo fmt --all -- --check
• deno fmt --check on all six modified Markdown files
• bash -n scripts/publish.sh
• git diff --check

Independent contract, controller, resource, and final-diff reviews found no implementation Blocker/High/Medium issue. The remaining blocker is intentionally the spec/rollout gate above.

No Phase 6+ work is included.