fix(sdk): snapshot index pane handles via resolved stable id

[grindset95]

Fixes #94

Problem

Slot handles from Session::pane(w, p) snapshot through the by-slot endpoint, which sends the visible (base-adjusted) pane index. The daemon resolver (pane_id_for_target -> window.pane(idx)) matches the raw index. When pane-base-index != 0 the lookup misses, the daemon returns InvalidTarget, and the SDK swallows it into a blank PaneSnapshot::default() with revision: 0. The by-id path is unaffected because the daemon maps the stable pane id back to the raw index internally.

Fix

Slot-handle snapshots now resolve the stable pane id and use the by-id endpoint, falling back to the legacy slot endpoint for daemons without by-id support. The documented stale-slot contract is unchanged: genuinely absent slots still return the default snapshot.

Scope note

The defect fires only when pane-base-index != 0. Revisiting the report with that understanding: under default config (base-index 1 / pane-base-index 1), the (0, 0) blank was a genuinely absent slot under literal indexing (correctly blank by contract), while (1, 1) was the live pane hitting this bug; it is repaired by this change.

Testing

• New regression test index_pane_handle_resolves_live_pane_under_nonzero_base_index: fails on unmodified main, passes with the fix, and fails again if the fix is reverted.
• Full pane_queries and list_panes suites pass; existing stale-slot and multi-window literal-index tests unchanged and green.
• cargo fmt --check and cargo clippy -- -D warnings on the changed targets are clean; the workspace build succeeds. One pre-existing lint in tests/armed_wait.rs reproduces on unmodified main with a newer local toolchain and is unrelated to this change.
• Independently verified end-to-end with the probe binary from the original report against a freshly built daemon in all three base-index configurations.

Possible follow-ups (maintainer call, out of scope here)

• The daemon-side by-slot resolver interprets PaneTarget.pane_index as raw while list-panes renders visible indices; making target resolution visible-index-aware would also fix other by-slot commands (send-keys, kill-pane, select-pane, resize).
• snapshot() on a handle that never resolves still returns Ok with the default grid; an explicit error variant (option 2 in SDK: index-based pane handles return silent all-blank snapshots (revision: 0) where by-id resolution succeeds #94) may be worth considering.