fix(web): preserve loaded history during streaming

[GoldenZqqq]

Summary

When the user is not at the bottom, streaming agent messages went through ingestIncomingMessages and were immediately front-trimmed to the normal visible window (~400). An expanded older-history window (~800 after Load older) could therefore lose the older page as soon as the agent streamed more output, so Load older looked broken or had to be repeated.

Changes:

1. Above-bottom agent merges no longer trim; they only merge into the current window.
2. setAtBottom(true) compacts back to the normal window when the user returns to the bottom.

Test plan

• cd web && bunx vitest run src/lib/message-window-store.test.ts (23 passed), including new case keeps loaded history while agent output streams above the bottom
• Manual: load older in a long session, stay scrolled up while the agent streams; older messages remain
• Manual: scroll back to bottom; window trims to the recent visible range and hasMore stays correct

Fixes #1071

[github-actions]

Findings

• [Major] Scrolled-up streaming window is now unbounded — why it matters: while the user is reading older history, every agent SSE merge appends into messages without any trim. This store feeds SessionChat/HappyThread directly, so a long-running stream can keep growing the rendered thread until the user returns to the bottom. The previous code kept the live window bounded; this PR should preserve the loaded older page without removing the cap entirely. Evidence web/src/lib/message-window-store.ts:1041.
  Suggested fix:

  const messages = mergeMessages(state.messages, agentMessages)
  const { kept, dropped } = trimPreservingQueued(
      messages,
      OLDER_LOAD_WINDOW_SIZE + VISIBLE_WINDOW_SIZE,
      'append'
  )
  const pending = filterPendingAgainstVisible(state.pending, kept)
  state = buildState(state, {
      messages: kept,
      pending,
      ...cursorUpdatesAfterAppendTrim(kept, dropped)
  })

Summary

• Review mode: initial
• One major issue found: above-bottom streaming now has no bounded window. Residual risk: the new test covers one streamed message, but not a long stream past the expanded-window budget.

Testing

• Not run (automation)

HAPI Bot

[github-actions]

[MAJOR] Scrolled-up streaming now appends into messages without any cap. Because this store feeds the chat thread directly, a long-running agent stream while the user is reading older history can keep growing the rendered thread until they return to the bottom. Preserve the loaded older page, but keep a bounded scrolled-up window.

Suggested fix:

const messages = mergeMessages(state.messages, agentMessages)
const { kept, dropped } = trimPreservingQueued(
    messages,
    OLDER_LOAD_WINDOW_SIZE + VISIBLE_WINDOW_SIZE,
    'append'
)
const pending = filterPendingAgainstVisible(state.pending, kept)
state = buildState(state, {
    messages: kept,
    pending,
    ...cursorUpdatesAfterAppendTrim(kept, dropped)
})