editor: slide activity rows to the newest window instead of the oldest

[sandersonstabo]

Why

In a long thread (VeriGuard, 381 activity items), tool calls stopped appearing after the first few commands of the live turn while prose kept streaming. project_activities sorted every retained row oldest-first and truncated to MAX_PROJECTED_FACTS = 256, so each replay re-picked the same oldest rows and the newest were never projected. The Forge had stored and delivered all of them (132 commands in the live run).

What

• activity_window(snapshot) = SCENE_MAX_ITEMS - snapshot.items().len(). The projection keeps the newest facts that fit and reports the rest as evicted.
• Replay removes registered derived facts that slid out of the window before upserting, so their room goes to live work instead of hitting the scene bound (which also swallowed errors).
• Fact ordinals count every projectable row, evicted ones included. A fact keeps its ordinal while the window slides; positional ordinals would collide (SceneConflict) and drop the newest row.
• ConversationStateController::derived_fact_ids / ConversationHost::derived_fact_ids expose the derived facts for that removal.

Tests

activity_slides_to_the_newest_window_and_frees_room_for_live_work fills a turn past the window, checks the oldest rows slid out and the newest are kept, then adds live work and checks it takes the freed room. It fails against the old projection.

cargo test -p artisan-frontend --test conversation_observation_projection: 21 passed.
--lib: 775 passed, 3 failed. work_group_header_row_paints_without_a_disclosure_wrapper, send_has_stable_identity… and harness_switch_animation_settles_without_rebuilding_catalog fail identically on master.

Follow-up, not in this PR: snapshots whose own durable items exceed SCENE_MAX_ITEMS are still refused, and older turns are never range-loaded.

🤖 Generated with Claude Code

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Summary by cubic

Fixes activity projection so the newest tool calls appear in long threads instead of being stuck behind the oldest retained rows.

• The projection now keeps the newest activity facts that fit the scene window and evicts the oldest.
• Replay removes derived facts that slid out before upserting, freeing room for live work.
• Fact ordinals count every projectable row, so a fact keeps its place as the window slides.

^{Written for commit 088d84a. Summary will update on new commits.}