fix(observer): align scroll-anchor ids with transcript display-block keys

[wpfleger96]

Summary

The observer feed's useAnchoredScroll hook was fed raw seq:timestamp ids while the inner AgentSessionTranscriptList rendered aggregated transcript block keys (turn:<id>, session-boundary:..., item ids). This id-namespace divergence caused the hook's floor writers to fire on every raw event against a DOM that barely changed, producing flicker and jump-to-tail during mid-history reading.

This PR fixes the divergence in both surfaces that wire useAnchoredScroll to observer transcript blocks:

1. Thread panel (AgentSessionThreadPanel) — derives display-block keys from raw events via the production chain (buildTranscriptState → buildTranscriptDisplayBlocks → getDisplayBlockKey), stabilized by value so the messages reference only changes when the ordered block-id sequence changes. View mode (raw vs transcript) folded into the reset key for clean re-initialization on toggle.

2. autoTail consumers (AgentSessionTranscriptList, used by profile live-activity cards) — the same disjoint-namespace bug existed here: the hook was fed TranscriptItem ids while the DOM rendered getDisplayBlockKey(block). Fixed by deriving block keys from the already-computed displayBlocks array, value-stabilized with useStableArrayShallow. Non-autoTail behavior (hook disabled via channelId: null) is unchanged.

Additional cleanup: raw-mode derivation skip (don't run deriveTranscriptBlockIds when showRawFeed is true), buildTranscriptState widened to readonly ObserverEvent[] (eliminating the cast in deriveTranscriptBlockIds), and the mid-history zero-write test strengthened to actually latch mid-history via onScroll.

useAnchoredScroll.ts is byte-zero — untouched.

Changes

• agentSessionTranscriptGrouping.ts — export deriveTranscriptBlockIds helper (production derivation chain); drop as ObserverEvent[] cast now that buildTranscriptState accepts readonly.
• agentSessionTranscript.ts — widen buildTranscriptState and buildTranscript to accept readonly ObserverEvent[].
• AgentSessionTranscriptList.tsx — derive block keys from displayBlocks via getDisplayBlockKey, stabilize with useStableArrayShallow, feed to useAnchoredScroll instead of raw items. Only active when autoTail is true.
• AgentSessionThreadPanel.tsx — skip deriveTranscriptBlockIds when showRawFeed is true; derive transcript block ids, stabilize, fold mode into reset key.
• useAnchoredScroll.observerScrollIds.test.mjs — mid-history test now latches mid-history via double onScroll (clear settle guard + latch anchor) with a fake [data-message-id] row, matching the pattern in the "new id mid-history" test.

What this fixes

• Mid-history reading is never yanked to tail (both thread panel and autoTail surfaces)
• At-bottom tail glue works without per-raw-event React floor writes
• Profile live-activity cards (autoTail consumers) no longer flicker
• Deterministic pin on feed open preserved (fix(desktop): unify observer feed scroll onto useAnchoredScroll #1825's win)
• Raw mode unchanged; unnecessary transcript derivation skipped in raw mode
• Archive load-older holds reading position

Fixes the v0.4.2 regression introduced by #1825.