tui: render pin tiles at the parser's current size (fix split+pin lag)

[edwin-zvs]

Problem

The TUI becomes extremely laggy when a split view and the pin strip are shown together — but it's fine with either alone.

Cause

A pinned session that's also visible in a split pane shares one cached vt100 parser per ItemHistory between the main render and the pin tile. replay resizes that parser to the requested dims, and a width change rebuilds it (re-feeds the pending chunk through a freshly-sized grid).

• The main/split render sizes the parser to the split pane's width.
• The pin tile then replayed the same parser at app.terminal_pane_size (the whole-view width).

Two different widths on the same parser each frame → two rebuilds per frame. Measured at ~45,000× the cost of a no-op resize on a long history (≈2 ms per session per frame). A few pinned-and-split sessions blow the frame budget.

The code already documented this exact thrash and "fixed" it by rendering the pin at the single main view size — which was safe when there was one main view. Split view gives each pane its own width, re-breaking the assumption.

stable width (no-op):        1 µs / 20 frames
alternating split/pin width: 45,146 µs / 20 frames   (~45000x)

Fix

The pin tile renders at the parser's current cached_dims() — the width the main/split render already set this frame — so its replay is a no-op resize and the parser is never rebuilt. Falls back to the main-view size only to seed a session with no cached parser yet (pin-only, never opened in the main view). cached_dims() is un-gated from cfg(test).

The invariant: not "render the pin at a fixed size" but "render the pin at whatever size the main render already set" — which holds for any number of split panes. See specs/0025-pin-tiles-share-the-main-parser-at-one-size.md.

Testing

• New regression test pin_tile_reuses_cached_size_to_avoid_split_thrash — asserts the cached-size reuse is >10× faster than the forced-width thrash (machine-verified the cause and the fix).
• cargo test -p agentd-cli pty_render (83) + pin (15) green; full workspace builds clean.
• Worth a real check on your end: open a split view with pinned sessions and confirm the lag is gone.

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