fix(grid): shift selection indices on step insert/remove

[mrcsin]

Problem

The grid caches the current selection as a list of step indices
(SelectedStepIndices). When a step was inserted or removed at or before a
selected step, the control's own selection model shifted to keep the same
items selected, but the cached indices were left stale. Consumers that read
the cache — copy, cut, delete, and the paste insert position — then operated
on the wrong steps.

The bug is a correctness/data-integrity defect. It is largely latent in the
current single-window UI, because every consumer command reselects after
mutating and undo/redo clears the selection, so no clickable gesture leaves a
stale cache for a later read. It becomes reachable the moment an "insert
above" command, a second window on the same recipe, or a consumer without a
follow-up reselect is added. The fix makes the invariant hold unconditionally.

What changed

RecipeGridSurfaceBase.OnMutation now maintains the cache itself instead of
relying on a control-level signal:

• A pure ShiftSelection(selection, signal) shifts the cached indices by the
  mutation signal — insert shifts survivors up, remove drops the removed set
  and shifts survivors down, a full rebuild clears the selection.
• The selection is snapshotted at the top of OnMutation and re-applied after
  the collection mutates, guarded by a sequence-equality check, so a
  mid-mutation push from the transposed list cannot double-shift.
• The shift is scoped to the four index-affecting signals; identity signals
  (including StepActionChanged, whose item reference is swapped in place)
  deliberately leave the cache untouched, so the existing reselect path governs
  them.
• The old drop-only ReconcileSelectionWithItems clamp is removed.

This works for both grids, including the canonical DataGrid, which exposes no
index-shift signal, and for the orientation-flipped surface whose view is
detached.

How it was verified

• 22 targeted SelectionShift tests across both grids: insert/remove before,
  after, and at the selection; multi-select block shifts; multi-insert +count
  and the multi-remove survivor offset; RecipeReplaced clears; ChangeStepAction
  non-clobber; and a consumer end-to-end test that deletes through the shifted
  cache and asserts the originally selected step is the one removed, by content.
• One transposed live-control test asserts the cache equals the control's own
  selection indices after a remove that pushes mid-mutation.
• Full suite green: 1466 passed, 0 failed.
• dotnet format --verify-no-changes clean; analyser clean on the changed file.

No clickable manual repro exists in the shipped single-window UI (see Problem);
the automated tests are the evidence.

Per-task commits before collapse

2585a8e docs(plans): archive stale-selection-shift plan
3095118 docs(plans): record exec branch and progress file
eedd215 test: cover multi-insert and multi-remove offset arithmetic
52a382c docs: note OnMutation selection-shift reconciliation
c33d74f test: verify acceptance criteria for selection shift fix
6d0eaa1 test: cover selection shift both grids, consumers, action-change
483e84d fix: shift cached selection indices on index-changing mutations
948d9ce test: red tests for stale selection index shift
cf0f9fa docs(plans): plan stale-selection-index shift fix

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