fix(terminal): gate the row-preserving skips on a capture, not the query flag

Review of the previous commit found the guard inverted: the three skips keyed
on `?full=1`, which is only what the client asked for. When the capture comes
back null — ENOBUFS, a timeout, a vanished pane, or a session with no mux at
all — the reply falls back to the byte history, which IS a stream of
successive frames and still needs stripping. Gating on the request returned it
whole: measured at 82KB against 4KB for the same buffer without `full=1`. A
direct-PTY session takes that path on every first selection, not only during
an outage. The skips now key on `isFullCapture`, meaning a capture arrived.

Three further defects the same review surfaced, all on this path:

Keeping the trailing rows is only sound when a cursor move follows to count
back up from them. On the two branches where the cursor query fails there is
no move, so the caret was left at the bottom of the pane — worse than before.
The cursor is now read first and settles both decisions together.

The move is relative rather than absolute. `CUP` numbers rows from the top of
the browser's screen, so it is only right while the browser's row count equals
the pane's, and `resizeWindow` does not wait for tmux, so a capture can be
taken before a requested resize applies. Measured against real tmux with a
browser four rows shorter than the pane: the absolute move lands on a blank
row, the relative one lands on the caret's row.

An all-blank pane no longer reads as content. Retaining trailing rows and
appending a move made it non-empty, and the caller treats non-empty as "replay
this", so a blank screen would have replaced real history — the downgrade
`_replayWouldShrinkBuffer` refuses, arriving from the server side where that
guard cannot see it.

The documentation claimed one line per screen row. `-J` joins a hard-wrapped
row into its logical line, so that is false whenever any row wrapped: measured
at 10 lines for a 12-row pane. Both entries now say what actually holds, and
the stale "NOT repositioned" contract in the mux interface is updated too.

Tests: the byte-history fallback is stripped, an empty capture leaves history
intact, and the extracted helpers are unit-tested directly rather than through
source-text matching. The slice window in the capture test is bounded at the
next method, having overrun into its neighbours.

src/web/routes/session-routes.ts

@@ -2603,6 +2603,12 @@ export function registerSessionRoutes(
         ? 'mux-full-history'
         : 'mux-visible'
       : 'history';
+    // What the three row-preserving skips below must key on. `isFullReload` is
+    // only what the CLIENT ASKED FOR: when the capture comes back null — ENOBUFS,
+    // a timeout, a vanished pane, or a session with no mux at all — rawBuffer
+    // falls back to the byte history, which is a stream of successive frames with
+    // no row alignment to protect and every reason to be stripped.
+    const isFullCapture = isFullReload && hasLiveMuxBuffer;
     let rawBuffer: string;
     if (liveMuxBuffer !== null && liveMuxBuffer.length > 0) {
       // Full-history capture is the RENDERED form of everything already in the
@@ -2656,7 +2662,7 @@ export function registerSessionRoutes(
     // border rather than its input line. Redraw-bloat stripping exists for a
     // byte stream of successive frames; a capture holds no successive frames.
     let strippedBuffer =
-      isFullReload || getCli(session.mode)?.capabilities.stripInkBloat === false
+      isFullCapture || getCli(session.mode)?.capabilities.stripInkBloat === false
         ? rawBuffer
         : stripInkRedrawBloat(rawBuffer);
 
@@ -2699,7 +2705,7 @@ export function registerSessionRoutes(
       // Skipped for a full reload: the banner sits at whatever row the pane has
       // it, and cutting to it would drop the blank rows above and move every
       // row up by that many.
-      const claudeMatch = isFullReload ? null : cleanBuffer.match(CLAUDE_BANNER_PATTERN);
+      const claudeMatch = isFullCapture ? null : cleanBuffer.match(CLAUDE_BANNER_PATTERN);
       if (claudeMatch && claudeMatch.index !== undefined && claudeMatch.index > 0) {
         let lineStart = claudeMatch.index;
         while (lineStart > 0 && cleanBuffer[lineStart - 1] !== '\n') {
@@ -2714,7 +2720,7 @@ export function registerSessionRoutes(
     // blank line is the pane's own first row and dropping it shifts every row
     // up by one.
     cleanBuffer = cleanBuffer.replace(CTRL_L_PATTERN, '');
-    if (!isFullReload) cleanBuffer = cleanBuffer.replace(LEADING_WHITESPACE_PATTERN, '');
+    if (!isFullCapture) cleanBuffer = cleanBuffer.replace(LEADING_WHITESPACE_PATTERN, '');
 
     const finishedAt = performance.now();
     reply.header(