Implement collapseBraid() per runtime spec

[flyingrobots]

Migrated from Method backlog

GitHub Issues are now the live work tracker. Repository docs remain Method evidence.

Field	Value
Source backlog	docs/method/backlog/v21.0.0/PROTO_strand-collapse-implementation.md
Archived source	docs/archive/backlog/github-issue-migration-2026-06-01/docs/method/backlog/v21.0.0/PROTO_strand-collapse-implementation.md
Original lane	v21.0.0
Original id	PROTO_strand-collapse-implementation
Original legend	PROTO
Original feature	merge-strands-worldlines
Original blocked_by	PROTO_strand-collapse-optic-for-causal-slicing, PROTO_local-site-object-for-neighborhoods

Original backlog card

Implement collapseBraid() per runtime spec

The strand runtime spec (strands-braids-runtime-spec.md §12) defines
collapse as:

collapseBraid(braid: BraidView, policy: CollapsePolicy)
  -> DerivedLane | Plurality | ConflictArtifact | Obstruction

Current state

• braidStrand() EXISTS — creates a braid view by composing read
  overlays from multiple strands over a common basis.
• collapseBraid() DOES NOT EXIST — no method that takes a braid
  view and derives a lane-level result under policy.
• analyzeConflicts() EXISTS — detects conflicts but does not
  perform admission or derivation.
• The braid-collapse-outcome-algebra.md defines the four lawful
  outcome types (Derived, Plural, Conflict, Obstruction).

What needs building

1. CollapsePolicy type — defines the rules for cell-by-cell
   admission (commutation evidence, join existence, plurality
   tolerance, obstruction conditions).

2. collapseBraid() method — cell-by-cell collapse per §12:

   • Single-claim cells: carry forward
   • Joinable cells: derive joined result
   • Irreducible plurality: preserve as Plural
   • Conflict: emit ConflictArtifact
   • Legality failure: emit Obstruction

3. Outcome types — runtime-backed classes per Paper VII:

   • DerivedLane (lane-level result from collapse)
   • Plurality (preserved plural claims)
   • ConflictArtifact (surfaced conflict with witness)
   • Obstruction (blocked with reason)

4. Observer collapse vs canonical collapse — §13 distinction:

   • Observer collapse = projection fact (what the observer sees)
   • Canonical collapse = admission fact (what becomes shared truth)
   • Never conflate them in the implementation.

5. admitLane() — separate from collapse. A derived lane is not
   canonical merely because it exists. Admission is the governance
   step that blesses it.

Alignment with Paper VII

Paper VII §3 (Braid Admission) is the theoretical frame. The runtime
spec §12 is the implementation contract. Both agree:

• A braid is NOT a merge
• Collapse does NOT mean "pick a winner"
• Irreducible plurality is a lawful outcome
• The witness must explain WHY the outcome is lawful

Graft dependency

Graft requested strand collapse work. This item provides the
runtime implementation that Graft (and Echo, Continuum) can consume.

Release home

Likely release home: v21, with possible seam work earlier.

The old v17.x or v18.0.0 target is stale. After 0037, this is clearly on
the plural/distributed side of the ladder:

• v19 should establish honest observer/admission seams
• v20 should make slice-first runtime execution real
• v21 should carry braid-collapse and common-basis plurality semantics