[Backend][Proposals] Move into a column with sparse positions throws at Apply (Insert past list end); preview releases the source WIP slot for it (from PR #3019 Codex review)

[Chris0Jeky]

From the Codex review of PR #3019 (issue #2926) at head fa62b03d04b6da287d6cca5f2e57b713a47d56c2 (inline comment 3992872287, P2). Source-traced, not runtime-reproduced. Non-blocking: the outcome is the pre-existing "approve passes, apply fails and rolls back" shape, never an over-limit column, and the underlying defect predates #3019.

Defect (pre-existing, Apply side)

When a proposal move targets a column whose card positions are non-contiguous (for example positions 0 and 2 after a middle card was deleted, or sparse positions from an import), OperationHandlerRegistry computes the append position as max+1 (3) and CardService.MoveCardAsync calls Insert(3, ...) on a two-item list, which throws. The proposal rolls back at execution.

Interaction with #3019

The preview projection treats such a move as executable and releases its source slot, so a following restore can pass approval on capacity the failed move never frees. Disposition is unchanged (the proposal cannot apply either way); only the failure moves from approve-time to apply-time for that combination.

Acceptance

• Make the executor append by list count (or normalise positions before insert) so a move into a sparse column succeeds; regression with positions 0 and 2.
• Alternatively, or additionally, have the preview validator refuse a move whose append position is invalid, keeping the source occupancy.
• Preview/apply parity test for the sparse-column move plus restore combination.

Related: #3020 (preview does not WIP-check create/move themselves), #3012 (conflict-detector projection omits lifecycle effects).

[Chris0Jeky]

[Claude lane claim v2]

• Lane: alpha-product-trust (proposal apply/preview seam)
• Issue: [Backend][Proposals] Move into a column with sparse positions throws at Apply (Insert past list end); preview releases the source WIP slot for it (from PR #3019 Codex review) #3025
• Base SHA: d7c2a4e (origin/main)
• Branch: issue-3025/sparse-column-move
• Owned paths:
  • backend/src/Taskdeck.Application/Services/Pipeline/OperationHandlerRegistry.cs
  • backend/src/Taskdeck.Application/Services/CardService.cs
  • backend/tests/Taskdeck.Application.Tests/Services/Pipeline/OperationHandlerRegistryTests.cs
  • backend/tests/Taskdeck.Application.Tests/Services/CardServiceTests.cs
  • backend/tests/Taskdeck.Application.Tests/Services/Pipeline/ProposalOperationContractValidatorTests.cs
  • docs/STATUS.md (one bounded line, if shipped behaviour changed)
• Scope: executor-side fix only (append by list count / clamp the insert index). Explicitly NOT
  taking on [Work model] Preview does not WIP-check create or move operations in a proposal #3020 (WIP-check for create/move in preview) or [Work model] Align proposal conflict projection with ordered lifecycle and active-state effects #3012.
• Status: claimed, implementing.

[Chris0Jeky]

[Claude lane release v2]
lane: alpha-product-trust
issue: #3025
PR: #3034
exact head: acf8b8e
result: merged
shipped outcome: a proposal move into a column with sparse positions now appends by the live card count instead of throwing at Apply; the Moved audit row records the effective position. Merge 44996ab; ci-required run 34659935662 green at the exact head; full backend solution green locally at the same head; one fresh-context review, no CRITICAL/HIGH, LOW findings triaged on the PR.
remaining residual: none tracked on this issue beyond the review triage recorded on the PR.
released paths/leases: OperationHandlerRegistry, CardService.MoveCardAsync and their tests.