fix(terminal): preserve scroll intent across keyboard resize, surface history truncation

Closes #259, closes #258. Both bottom out in the same gap: nothing tracked
whether the user was following live output or reading history.

#259 — the keyboard path forced the terminal to the bottom unconditionally
(onKeyboardShow/onKeyboardHide passed scrollToBottom:true, applied with no
check), so opening the keyboard while scrolled up yanked the user down. The
settle cycle now captures intent on its FIRST event, before any fit() has
reflowed the buffer, and returns to that anchor when the user was reading.
A later capture would read an already-moved viewportY, which is why the
capture point matters. The param is renamed restoreScroll to match.

Separately, flushPendingWrites gated viewport preservation on
_hasRecentUserScrollUp(), a 1500ms decay window, so a user who scrolled up and
then actually READ for longer lost protection mid-read. Being scrolled up IS
the intent however long ago it was expressed, so it now keys off position.
The recency window stays as a race guard on the sticky scroll-to-bottom.

The full-history repull already held the user's place and is unchanged.

#258 — truncation was reported by a grey line written INTO the terminal
("earlier output truncated"), which scrolls away with the output it describes,
cannot be acted on, and said the same thing whether the rest was one click away
or gone forever. The server set one `truncated` boolean at two sites meaning
opposite things, and the client discarded fullSize and source entirely.

The route now reports truncationReason ('tail' = intentional partial replay,
the rest is retained; 'capped' = the byte ceiling dropped it) plus
retainedBytes, and 'capped' is not downgraded by a later tail cut. The client
renders a dismissible banner outside terminal output with three honest states:
recoverable (offers Load full history), at-ceiling, and exhausted. The Load
button forces past the scroll cooldown but NOT past _replayWouldShrinkBuffer,
which still refuses a downgrade for repaint-mode panes.

The banner is an overlay, not a flex child: FitAddon derives rows/cols from the
terminal parent's computed height, so occupying real layout space would SIGWINCH
the CLI on every truncation-state change.

Verified in a real browser on the 7 skins: banner text and button clear 4.5:1
contrast on all of them, and terminal height is byte-identical with the banner
shown. The first cut used --bg-elevated and --accent-muted, which do not exist,
so light skins rendered a hardcoded dark bar under dark text; it now uses only
tokens every skin redefines.

test/terminal-scroll-intent.test.ts lives outside test/mobile/ deliberately —
that suite is excluded from test:ci, so a guard placed there is invisible to CI.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

src/web/routes/session-routes.ts

@@ -2292,6 +2292,14 @@ export function registerSessionRoutes(
     }
     const fullSize = rawBuffer.length;
     let truncated = false;
+    // WHY the reason and not just the boolean (#258): `truncated` is set at two
+    // sites that mean opposite things to a user. 'tail' is an intentional
+    // partial replay and the rest is still retained, so a `full=1` pull recovers
+    // it. 'capped' means we hit the byte ceiling — and on a full-history capture
+    // that is already everything tmux holds, so the oldest output is genuinely
+    // out of reach rather than one click away. Collapsing both into one flag is
+    // why the UI could only ever say "truncated for performance".
+    let truncationReason: 'capped' | 'tail' | null = null;
     let cleanBuffer: string;
 
     // Cap the payload EARLY — before the regex normalization passes below run
@@ -2302,6 +2310,7 @@ export function registerSessionRoutes(
     if (terminalBufferMaxBytes > 0 && rawBuffer.length > terminalBufferMaxBytes) {
       rawBuffer = rawBuffer.slice(-terminalBufferMaxBytes);
       truncated = true;
+      truncationReason = 'capped';
       const capNewline = rawBuffer.indexOf('\n');
       if (capNewline > 0 && capNewline < 4096) {
         rawBuffer = rawBuffer.slice(capNewline + 1);
@@ -2335,6 +2344,9 @@ export function registerSessionRoutes(
       // Banner is near the top and gets discarded by tail anyway.
       cleanBuffer = strippedBuffer.slice(-tailBytes);
       truncated = true;
+      // 'capped' already means the oldest bytes are gone for good; a tail cut on
+      // top of it does not soften that, so the stronger reason wins.
+      truncationReason ??= 'tail';
       // Avoid starting mid-ANSI-escape: find first newline within the first 4KB
       // and start from there. This prevents xterm.js from parsing a partial escape
       // sequence which corrupts cursor position for all subsequent Ink redraws.
@@ -2365,6 +2377,10 @@ export function registerSessionRoutes(
       status: session.status,
       fullSize,
       truncated,
+      truncationReason,
+      // `retainedBytes` is what this response actually carries; `fullSize` is
+      // what existed before the cut. The gap is what the indicator reports.
+      retainedBytes: cleanBuffer.length,
       source,
     };
   });