devindex: remove the locked-column test config before release

Context

Operator-proposed (2026-06-12 wrap-night session, converged in dialogue): the locked columns in devindex were added for testing only during the multi-body grid work. With the testing purpose now served elsewhere, they should come out of the public demo before the v13 release.

Release classification: ON the board — touches the release's public demo surface; operator proposed pre-release timing.

The Problem

devindex ships with locked-column config whose only remaining effects are negative:

• Mobile portrait is arithmetically broken: from tonight's live trace data, the locked-start region alone ends at x≈387px, locked-end adds ~100–150px — ≥490px of immovable columns against a ~390px portrait viewport leaves the scrollable center with negative space. (Operator's untested guess, closed by arithmetic.)
• It showcases the configuration where the open grid defect family (#12883 / #12929 / #12930 / #12932) is most visible — undercutting the release notes' deliberately honest Body/Grid scoping ("shipped foundation, not completed multi-body architecture").

The Architectural Reality

• Test coverage does NOT depend on this config (the decisive evidence, operator-pointed): test/playwright/e2e/GridBigDataMultiBodyNL.spec.mjs runs against /examples/grid/bigData/ and creates its own locked topology at runtime via the Neural Link fixture (app.setProperties(colId, {locked: 'start'|'end'})), asserting three bodies + cross-body selection sync + worker-state truth. Dynamic locking exercises the onColumnLockChange transition — deeper coverage than any static config provides. Ada's #12807 grid whitebox-e2e oracle adds the column-distribution surface.
• The proper home for responsive lock behavior is the engine feature tracked separately (see Related), not a hardcoded demo config.

The Fix

Remove the locked config entries from the devindex grid columns (config-only change; the operator already validated the no-locks state live during tonight's debugging). Note in the PR: #12883 (locked-end re-home, @neo-fable) describes its repro against devindex-with-locks — its repro env note should point at the bigData example / dynamic-locking fixture after this lands.

Acceptance Criteria

• No locked column config remains in the devindex grid.
• devindex renders + scrolls correctly with the single-region layout (smoke check).
• #12883 gets a repro-env pointer comment (bigData example + NL dynamic locking) so its repro recipe survives the removal.

Out of Scope

• Responsive/breakpoint-driven lock states (companion enhancement ticket, post-release).
• The grid defect family fixes themselves (#12929 / #12930 / #12932 / #12883).

Related

• Coverage home: GridBigDataMultiBodyNL.spec.mjs (dynamic locking) · #12807 (whitebox-e2e oracle).
• Defect family context: #12883, #12929, #12930, #12932.
• Companion: responsive locked-column states (filed alongside this ticket).

Live latest-open sweep: checked latest 15 open issues at 2026-06-12T00:18Z; no equivalent found. A2A in-flight sweep: clean.

Lane note: filed unassigned — claimable; small config lane, suitable for any peer's next slot (operator merge-gates the PR as always).

Origin Session ID: e6b095bb-4d88-4aeb-a0ec-7d47e0f8f7ce

Retrieval Hint: "devindex remove locked columns release demo mobile portrait viewport"