saikai 0.6.0 — faithful, cheap pane rendering

Live panes render a real terminal, and this release is mostly about making that
faithful and cheap on Windows Terminal: a pane no longer redraws itself wholesale on
every chunk, a frame can no longer mix two states of the child's screen, and the
widths in the model now match the widths on screen.

Fixed

• Rows in a pane no longer land out of place. render_line took the pane lock
  once per row while Textual calls it in a per-row loop, and the reader installs a
  whole child frame atomically under that same lock — so one composited image could
  carry rows from two pyte generations (measured: 40 of 40 render passes torn). The
  positions were always right and the contents were stale, which is why nothing in
  the output stream explained it. The visible grid is now pinned once at the frame
  boundary and read lock-free.
• Emoji no longer shift the rest of their row. ⚠ is one column but ⚠+VS16 is
  an emoji-presentation sequence that Rich — so Textual, so the terminal — draws in
  two, and the zero-width merge left the cluster owning one. Every glyph after it sat
  a column right and the row's tail spilled past the pane. Measured on a captured
  session: the rows carrying ⚠️ handed Textual a 98-cell row for a 97-cell pane.
• The pane no longer oscillates while scrolling. pyte erases per cell, so
  overwriting half a double-width glyph left the other half behind — a state no
  terminal can display. Rendering it forced a choice between eating a character and
  shifting the row, and a redrawing child alternated between the two. saikai now
  erases the whole glyph when either half is overwritten, as a real terminal does.
• Copying with the mouse no longer corrupts the pane. The same per-row read let
  _scroll and the freeze snapshot flip mid-frame, mixing pinned and live rows.
• The session list no longer jumps back while you scroll it. Three separate places
  moved the viewport during a rebuild: the rebuild itself (clear() zeroes scroll_y),
  a deferred restore that re-applied a pre-wheel position a frame later, and
  move_cursor(scroll=False), which did not actually prevent scrolling. The user's
  scroll position is now owned state rather than a snapshot guarded by a timer —
  measured on the real app under constant rebuilds: 0 backwards jumps and full travel,
  where before most of the travel was lost.
• A resize that changes nothing is ignored. Textual re-posts Resize on any
  relayout, and each one dropped the pane's scrollback offset, dirtied every pyte line
  and sent the child SIGWINCH. A real session logged seven identical 36x97 resizes
  in eight seconds.
• The native cursor no longer flickers over the list. Frames are bracketed in DEC
  2026 synchronized output on Windows (Textual's Windows driver never probes for it),
  the list's mouse-hover repaint is gone, and the periodic anchor no longer re-asserts
  ?25h on every tick.
• IME anchoring follows the caret. The anchor rides the repaint, freezes while the
  child is mid-frame, settles on the busy→idle transition, re-anchors when leaving
  scrollback and after a resize, and heals a drifted cursor-visibility state. The pane
  presents itself as Windows Terminal, keeping WT_SESSION so claude tracks the caret.
• Terminal queries are answered like a terminal. Primary and secondary DA, DECRQM
  with real mode state, DSR at the position the child asked from, in the order asked,
  once per split unit; DCS is scrubbed before pyte without leaking payloads or opening
  a deletion seam.
• The PTY write path never blocks the UI thread. Oversized writes go to the pane's
  writer thread, the input queue is bounded, a refused write is logged instead of
  vanishing, and a writer that fails to start no longer swallows the data it was given.
• Panes flagged "needs input" again for claude's resume and permission gates on the
  alt screen.
• Quitting takes a deliberate double press. A single Esc (or Ctrl+C) on the
  list quit saikai outright — too easy to fire by reflex (Esc interrupts claude in a
  pane; Claude Code itself exits only on a second Ctrl+C). The first press arms and
  shows a hint; a second within ~2.5s quits, and any other key disarms. Esc still
  leaves search / dropdown / pane → list as before.

Changed

• A pane repaints the rows that changed, not all of them. pyte tracks per-line
  damage and saikai never read it; a spinner frame touches one or two rows out of
  thirty-six. Measured per pane during an agent-mode storm: 115–670 ms of UI thread per
  second before, 8–33 ms after. The traps this has to survive are listed in the
  rendering invariants — a cursor move dirties nothing, resize leaves out-of-range
  indices, a child scroll dirties everything.
• The web mirror no longer forces a full-screen relayout per frame. Its
  overflow-recovery repaint was the only refresh(layout=True) in the repository and
  fired on essentially every drain during a storm; it is now rate-limited, gated on a
  connected viewer, and asks for a repaint rather than a relayout. The statusbar also
  stopped requesting a layout pass on every update.

Added

• SAIKAI_DIAG=1 captures the whole display picture in one run — per-pane child
  bytes, what saikai wrote to the terminal, per-second render accounting, a scroll /
  focus / resize trail, rows whose rendered width disagrees with the pane (with the
  row's text), and an automatic model dump at the first such row. One directory per
  launch. SAIKAI_FULL_REPAINT=1 and SAIKAI_NO_SYNC=1 isolate the two riskiest
  behaviours in a single variable.
• Documented rendering invariants and a debugging guide.
  ARCHITECTURE.md lists the five
  invariants with the symptom each produces when it breaks;
  DEBUGGING.md covers reading the diagnostics, replaying a capture
  offline, attributing a defect to saikai or to the child, and what each instrument
  cannot distinguish.

Known limitations

• East Asian Ambiguous characters (①, and others) can overhang their cell. Their
  width is not defined by the standard: the terminal advances one column while a CJK
  fallback font may draw two. This reproduces with claude in Windows Terminal without
  saikai, and inside saikai the model and the renderer agree on every one of the 639
  distinct non-ASCII characters in a captured session — so there is nothing for saikai
  to fix here.
• claude's own transient renders show through. Its spinner cycles, and its
  transcript briefly re-renders at a shifted offset; saikai reproduces the child's
  screen faithfully, including those.

Releases 0.4.0 – 0.5.2 were tagged without CHANGELOG entries. This entry covers the
work since 0.5.2 only; git log v0.3.0..v0.5.2 is the record for that gap.