Web UI: tail-paginate session transcripts with a progress banner

[edwin-zvs]

Problem

Opening a session in the web UI used to fetch the entire transcript in one session.transcript round trip and render every event in a single synchronous main-thread loop. For an ultracode-shaped session (1.5 GB transcript on disk, ~3.9M events) that meant several seconds of wire wait, then a UI freeze while every event parsed and built DOM. The only feedback was a static skeleton shimmer that appeared after 150 ms and never updated.

Fix

Tail-first: the first round trip returns just the most recent N events. Older pages then fill in upward without blocking the UI.

Server

• TranscriptParams gains tail: Option<usize>. When set, from / limit are ignored and the response is the last N events.
• Storage::read_transcript_tail(id, n) seeks backward in 64 KiB chunks until it has at least n + 1 newlines (so a partial first line never chops a real event), then parses only those lines. O(n × line size), not O(file size).
• SessionManager::transcript takes total from the live transcript_count atomic, so even a multi-GB transcript responds with bounded latency.

Web UI (crates/daemon/assets/index.html)

• Fetches { session_id, tail: 500 } first and renders the tail immediately, scrolled to the bottom — the user sees the live tail almost as fast as the previous skeleton would have appeared.
• If total > tail.length, pins a sticky progress banner ("Loading older history… 1,200 / 12,400 (10%)") to the top of the pane and walks forward from from: 0 in 500-event pages, prepending each batch above the tail. Scroll position is preserved relative to the bottom so prepends never make the visible content jump.
• Yields between pages with requestAnimationFrame so the page stays responsive.

Client::transcript keeps its current shape (passes tail: None) so the zarvis / MCP / TUI Rust callers don't need to opt in.

Test

• Storage: 5 unit tests on read_transcript_tail — last-N order, chunk-boundary safety (lines longer than the chunk size), n > total, missing file, n = 0.
• Session manager: transcript_tail_returns_last_n_with_live_total asserts total comes from the live counter and the slice is the right tail.

cargo test -p agentd --bin agentd   # 60 passed (55 prior + 5 new)
cargo build                          # clean

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