Make snapshot membership loss attributable: scroll clip authority + ejection dispositions

[thymikee]

Purpose

snapshot -i lost every list row after a row holding selectable text (#2214 in 0.21.0, patched for that
one type by #2740). The type-specific fix closed one instance. The class stays open: presentation still
finds a scroll indicator's owner by walking up past its parent, so any host that scrolls but publishes
as a non-scroll type (WebView, a map view, a paged cell) misattributes its indicator to the enclosing
list and clips that list to the host's band. Measured: a synthetic Element(58) row with its own indicator
drops Reply on origin/main.

Design record: ADR 0026 — Scroll Clip Authority: Ownership Is the Parent Edge (proposed, #2755).

Required behavior

Step 1 — ownership is read from the parent edge. The owner of an indicator is its parent, and a band
derives only when that parent is a scroll type. Delete the ancestor walk and isTextViewType from
packages/capture-kit/src/ios-snapshot-engine/scroll.ts. Evidence already collected on a scratch
branch: pnpm test:unit → 10,959 passed / 1 skipped with this substitution, and the Element(58) shape
above stops dropping rows. Land it with the red→green cases from step 2.

Step 2 — extend the differential to the runner stage. assertProjectionSubsets
(packages/capture-kit/src/ios-snapshot-engine/properties.test.ts) asserts interactive ⊆ regular ⊆ raw
but builds acquired inputs only; the Swift differential drops interactive cases. Add a stage: 'presented'
arm plus authored cases for a TextView row (#2740's shape) and a WebView row. Each must fail on
origin/main first, per docs/agents/testing.md. Assert source-level membership, not literally
interactive ⊆ full: the full projection skips semantic compaction.

Step 3 — survey real captured trees for under-clipping. Removing the walk can lose a band when a
real tree places an indicator under a labelled wrapper instead of directly under its scroll view, letting
content scrolled under the chrome survive — the leak class #1784/#1797 removed. Survey captured runner
trees before landing step 1, and record the count of indicators whose parent is not a scroll type. If the
shape is real, decide on evidence whether to skip unlabeled Other wrappers — do not add the tolerance
speculatively.

Step 4 — pair the two eligibility lists, and settle ScrollArea for macOS.
REGULAR_ELIGIBLE_TYPES (ios-snapshot-engine/projection.ts) and eligibleInteractiveTypes
(apple/snapshot-presentation/Sources/AgentDeviceSnapshotPresentation/SnapshotPresentationProjection.swift)
are one fact in two languages and no test currently references either. They are what makes the parent edge
survive projection: an ineligible wrapper is re-parented to the nearest presented ancestor, which is the
scroll host. Add a shared-table or dual-assertion guard. Note the pair excludes scrollarea while
isScrollableSnapshotType includes it: the iOS runner never emits that type, but macOS desktop capture
reaches these rules (snapshot-desktop-surface.ts → ios-snapshot-runtime.ts → publishIosSnapshot) and
its helper emits ScrollArea. Decide whether macOS needs it eligible before the rule is reused there —
do not resolve it by editing a set for iOS's sake.

Step 5 — inventory ejections, then make them typed. ~23 suppressNode call sites across ten rules
(rows, web, transitions, noise-search, noise-redundancy, noise-structural, noise-overlay,
noise-viewport, action-shelf, scroll); at least two also rewrite rects. Extend the existing
presentedIndexesBySourceIndex (ios-snapshot-engine/semantic-index.ts) into a typed disposition — every
source ends presented with representatives, or removed with at least one reason. If eject requires a
reason the ledger is complete by construction, so no separate gate is needed. Internal evidence only: no
suppression vocabulary on the snapshot wire.

Observable completion conditions

• A stage: 'presented' case for both the TextView and WebView row shapes fails on origin/main and
  passes with the change.
• No band can derive from anything other than a parent that is a scroll type; grep proves no ancestor walk
  remains in scroll.ts.
• The wrapper survey result is recorded on this issue with tree counts, and any tolerance it justifies
  carries a captured-tree fixture.
• Eligibility parity is enforced, not asserted in prose, and the ScrollArea question is answered for
  macOS surfaces — either decided with a case, or recorded as out of scope with the reason.
• pnpm check:affected --run on the exact pushed head, plus the engine conformance and differential suites.
  New ownership cases join contracts/fixtures/ios-snapshot-engine-conformance.json; they join the Swift
  fixture only once Swift implements the same behavior.

Non-goals

• A producer-reported ownership field or capture-local index remapping: the parent edge already carries it.
• Producer-computed visible bands.
• A scroll-capability matrix, or adding TextView/WebView to a universal scroll-container set.
• A rule-graph/effect framework, or a two-API reshape/eject split before step 5's inventory.
• Treating the unexplained 74 → 67 node delta as an acceptance baseline without a captured artifact.

Dependencies and sequencing

• Blocked by: maintainer decision on ADR 0026 (docs(adr): scroll clip authority — ownership is the parent edge #2755).
• Steps 1 and 2 land together. Step 3's survey gates step 1's merge. Step 4 is independent. Step 5 last.
• runner-presentation.test.ts pins the band contract, so the band must not be deleted to dodge the
  problem.

Related: #2214 (introduced), #2740 (type-specific fix), ADR 0004 §host-side ownership boundary and the
#2638 regression note ("an absent fact is not a negative answer").