fix: expand Ink partial clears to prevent ghost content in screenshots

Ink (React CLI framework) clears its output using repeated EL2+CUU1
sequences, one per previously-drawn line. When /clear resets Ink's
internal line counter, the next frame only erases a few lines instead
of the full previous output. In a real terminal the old content is in
scrollback and invisible, but pyte's fixed-size screen retains it,
producing ghost content (e.g. duplicated prompts) in SVG screenshots.

Added AltScreen.expand_clear_sequences() which detects runs of 3+
EL2+CUU1 pairs that don't reach row 0 and extends them to erase all
lines up to the top of the screen. Both DockerExecSession and
TerminalSession call this before feeding data to pyte.

Also made on_session_end() idempotent (contextlib.suppress KeyError)
to prevent a race when close_session() and natural session exit both
call it.

Added docs/ink-clear-fix.md with root cause analysis, byte-level
explanation, and reproduction script.

docs/ink-clear-fix.md

@@ -0,0 +1,110 @@
+# Ink Partial Clear Fix
+
+## Problem
+
+When CLI applications built with [Ink](https://github.com/vadimdemedes/ink) (React for terminals) — such as GitHub Copilot CLI — execute a `/clear` command, the resulting screenshot shows **ghost content**: old conversation lines persist above the fresh prompt.
+
+The real terminal displays correctly, but the pyte-based screen buffer used for SVG screenshot generation retains the stale content.
+
+### Example
+
+Before `/clear`, the screen has 30 lines of conversation. After `/clear`, the user sees only a fresh 6-line prompt, but the screenshot shows:
+
+```
+Row  0: [old prompt header]          ← ghost content
+Row  1: [────────────────]
+Row  2: [❯ hello]
+...
+Row 20: [● old response text]        ← ghost content
+Row 21: [/workspace[main]]           ← fresh prompt (duplicated)
+Row 22: [────────────────]
+Row 23: [❯  Type @ to mention files]
+Row 24: [────────────────]
+Row 25: [shift+tab cycle mode]
+```
+
+## Root Cause
+
+Ink uses a **line-by-line erase** pattern to clear its previous frame before drawing the next one:
+
+```
+ESC[2K  (EL2 — erase entire line)
+ESC[1A  (CUU1 — cursor up one row)
+```
+
+This pair is repeated once per line of the **previous** frame. Ink tracks how many lines it rendered and erases exactly that many before redrawing.
+
+When `/clear` is issued:
+
+1. Ink resets its internal "previous output height" counter to 0.
+2. On the next render cycle, Ink erases **0 old lines** (counter is 0), then draws the fresh prompt (~6 lines).
+3. The subsequent render erases 6 lines (the prompt it just drew), redraws — correct from now on.
+
+In a real terminal, the old content has already **scrolled into the scrollback buffer** and is invisible. But pyte's fixed-size `Screen` keeps all content in the visible buffer. The 6-line erase only clears rows at the bottom, leaving rows 0–24 with orphaned old content.
+
+### Byte-level example
+
+Cursor at row 30 after rendering a full conversation:
+
+```
+Frame N (normal):   EL2+CUU1 × 28 (clears rows 30→2)  + redraw 28 lines  ✓
+/clear resets counter
+Frame N+1 (broken): EL2+CUU1 × 6  (clears rows 30→24) + redraw 6 lines   ✗
+                    Rows 0–23 still contain old content!
+```
+
+## Fix
+
+`AltScreen.expand_clear_sequences()` in `alt_screen.py` pre-processes incoming terminal data before it reaches pyte. It detects runs of 3+ `EL2+CUU1` pairs and, if the run doesn't reach row 0, extends it with additional pairs so the erase covers all lines from the cursor position up to the top of the screen.
+
+```python
+# Before fix: 6 pairs clear rows 30→24, leaving 0–23 dirty
+data = b"\x1b[2K\x1b[1A" * 6
+
+# After expand_clear_sequences(): 30 pairs clear rows 30→0
+data = screen.expand_clear_sequences(data)
+# Now contains 30 pairs of EL2+CUU1
+```
+
+Both `DockerExecSession._update_screen()` and `TerminalSession._update_screen()` call this method after C1 normalization and before feeding data to `pyte.ByteStream`.
+
+### Why this is safe
+
+- The fix only triggers on runs of **3 or more** `EL2+CUU1` pairs (normal editing uses 1–2 at most).
+- It only **adds** additional erase operations for lines that would already be empty in a real terminal (they scrolled into scrollback).
+- The extra erases are no-ops if the lines are already blank.
+- Short runs (< 3 pairs) and runs that already reach row 0 are left unchanged.
+
+## Reproducing
+
+```python
+from webterm.alt_screen import AltScreen
+import pyte
+
+screen = AltScreen(132, 45)
+stream = pyte.ByteStream(screen)
+
+# Draw 27 lines of content
+for i in range(27):
+    stream.feed(f"Content line {i}\r\n".encode())
+
+# Ink /clear: only erases 6 lines
+partial_clear = b"\x1b[2K\x1b[1A" * 6 + b"\x1b[2K\x1b[G"
+
+# Without fix: old content remains on rows 0–20
+# With fix: all rows are cleared
+expanded = screen.expand_clear_sequences(partial_clear)
+stream.feed(expanded)
+
+# Draw fresh prompt
+stream.feed(b"Fresh prompt\r\n")
+
+non_empty = [line for line in screen.display if line.strip()]
+assert len(non_empty) == 1  # Only "Fresh prompt"
+```
+
+## Related
+
+- `WEBTERM_SCREENSHOT_FORCE_REDRAW` env var — sends SIGWINCH to force app redraw before screenshots, but doesn't fix this issue since Ink still only erases its tracked line count.
+- `docs/tmux-da-response-filtering.md` — another terminal compatibility fix.
+- pyte's known limitations with partial screen clearing are noted in `README.md`.