fix(desktop): hide non-contiguous islands until scrollback heals them

[tlongwell-block]

Stacked on #1483. Fixes the pop-in Tyler hit while hammering the stack: scrolling up sometimes rendered the beginning of a day before its end/middle, which then popped in above the reader.

Mechanism

Out-of-band merges (useLoadMissingAncestors fetching thread roots by id, useThreadReplies fetching subtrees) splice old events into the cache marked nonContiguous. #1483 made the pager ignore them when anchoring its cursor — but nothing filtered them from the render. An island (typically a day-leading thread root) painted instantly at its day position; the rest of that day only arrived when contiguous paging caught up.

Fix

Hide marked events from the main timeline until paging heals them:

• TimelineMessage carries the nonContiguous mark through formatTimelineMessages
• buildMainTimelineEntries skips marked entries — the single seam both ChannelPane and TimelineMessageList render through. Island replies still count toward a visible parent's thread summary (the thread index sees the full cache)
• countTopLevelTimelineRows mirrors the filter, so the pager's row floor counts only rows that actually render
• the channel unread marker skips hidden islands, so the pill/New-divider never anchor on an invisible row

Healing is unchanged: when contiguous paging reaches an island, the unmarked copy wins the merge and the whole day appears in one commit, in order. Thread panels are unaffected (they derive from the full cache, not the entries).

Validation

• 3 new regressions, all red on fix(desktop): stop out-of-band ancestor merges from poisoning the scrollback cursor #1483's head / green here: island hidden pre-heal + visible post-heal, hidden islands excluded from the row floor, mark propagation through formatting; plus island-reply-in-summary coverage
• Full-dataset e2e (real 2,980-event #buzz-bugs export): at every paging commit, the oldest rendered row never precedes the contiguous frontier (no pop-in), and the final render contains all 206 expected top-level rows (nothing lost)
• desktop suite 1504/1504, pnpm check clean, tsc --noEmit clean

[tlongwell-block]

Independent delta review clean at 1bc8e24accddfd022d65e765b0807afa9f485869.

Verified:

• deep-link/search events enter the render list through ordinary mergeMessages, unmarked; a same-ID route target also replaces a cached marked island in the resolved render list;
• an island thread head and marked descendants remain available through buildThreadPanelData while buildMainTimelineEntries hides only their main-timeline rows; the exact shape is now pinned by 1bc8e24a;
• pager row-floor and channel unread marker count the same visible top-level set as the render seam;
• contiguous history still heals a marked event through last-copy-wins without a downgrade race.

Validation on the exact PR head: desktop 1505/1505, pnpm check, pnpm typecheck, and git diff --check all clean.

Scores: Minimalness 9/10, Elegance 9/10, Correctness 9/10.

GitHub credentials resolve to the PR owner, so I cannot submit a formal approval; this comment is my approval-equivalent verdict. Manual scrollback remains the final UX gate.