Add pure client-local native pane layout model

[h0x91b]

Hi, this is Claude (the AI assistant working on this branch).

Small independent prerequisite for LAY-005 on the tmux-removal roadmap (parent Seq 1141). Adds a standalone, pure, import-free state model for a future native multi-pane terminal, without touching the renderer, terminal host, registry, the Seq 1254 adapter, SplitTree, or tmux.

What

New module src/shared/native-terminal-client-layout/:

• Models each client's view of the shared, ordered pane set plus its own client-local focus and optional zoom ({ paneIds, focusedPaneId, zoomedPaneId }).
• reconcileClientPaneLayout folds a fresh observation of the shared pane set into a client's state deterministically: a still-present focus/zoom is kept; a removed zoom target clears zoom; an invalid/missing focus falls back to the nearest surviving pane (next in the old order, else previous, else first); an empty set has null focus. Invariant focus === null ⇔ no panes, verified by validateClientPaneLayout.
• Focus and zoom are orthogonal overlays; every function is pure, immutable, and returns the same reference on a no-op.

Why it's safe

• No runtime side effects, no imports, and no PTY dimension anywhere in the module — proven structurally (state has exactly three keys) and by a source sentinel test that rejects imports and pty/cols/rows/resize/dimensions tokens.
• Local focus/zoom mutations keep the shared pane-list reference identical (result.paneIds === layout.paneIds) and never mutate the caller's inputs.
• Two clients reconcile independently over the same shared pane set (observer isolation).

Tests & docs

• 34 deterministic unit tests in src/mainview/__tests__/native-terminal-client-layout.test.ts (mirroring the split-tree test placement) covering 0/1/2/6 panes, focused-pane removal, zoomed-pane removal, reorder, and observer isolation.
• Decision record decisions/164-client-local-native-pane-overlay.md documents the orthogonal-overlay invariant and the deterministic neighbor fallback.
• No visible behavior change.