Unify the Each reconciler's keyed/positional paths into one planner + executor

[khusmann]

Unifies the Each reconciler's keyed and positional paths into a single pure planner and one mode-agnostic executor.

This PR landed in two stages on the branch:

1. Extract pure planners — pulled the decision logic (which entries to remove/add/keep/rebuild) out of the observer into pure functions taking shape signatures as data.
2. Unify the two paths (this is the headline change) — collapsed the keyed/positional fork into one planner + one executor.

The core insight

Positional Each is keyed Each whose id is the row position. With ids as stable strings — the key in keyed mode, as.character(index) in positional mode — every mode difference collapses to set algebra over old_ids / new_ids.

What changed (all in mount.R)

• One planner replaces two. plan_reconcile(old_ids, new_ids, old_sigs, new_sigs) returns removed/added/kept, a derived order (or NULL), and a build_index mapping added ids to their positions. No session, no build_entry, no DOM messages.
• Derived order policy. order is computed from the client's natural insert order (survivors in old order ++ added at the tail) and sent only when it differs from new_ids — no mode flag. This reproduces both modes' historical behavior and, as a free correct improvement, omits order on keyed tail-appends (smaller payloads). No irid.js / wire change required.
• Mode-agnostic executor. run_reconcile_plan() does teardown → build → irid-mutate → mount → reposition against a unified container. The pos_rv guard makes kept-repositioning a no-op for positional entries.
• The observer fork shrank to one line — the new_ids computation (as.character(by(x)) vs seq_along). Everything after is shared.
• Container unified from a dense-list / sparse-map split into a single named map keyed by string id, plus an ordered current_ids vector.
• build_entry stays mode-aware — its late-bound-key vs captured-index value access is the identity model, not incidental duplication.

Tests

Planner unit tests exercise the decision logic directly with plain data, including the derived order policy: append-no-order, reorder-order, mid-list-rebuild-order, keyed-tail-append-no-order, shrink-no-order, and build_index. The mount-based integration tests (the reconciler guardrail) pass untouched. 706 tests, 0 failures, plus a live browser check of the keyed reorder/focus path.

Follow-up

The client currently applies order with O(n) DOM moves; a client-only O(moved) optimization is tracked separately in #23 and deliberately decoupled from this change.