feat(palace): add ActivityEvent sink on PalaceBuilder (jcode shape) (mp-migration 5/8)

[quangdang46]

Summary

Add the API shape for activity streaming — the type, the builder method, and the storage slot. The jcode adapter can already register a sink; emit-calls from the runtime are scoped to a follow-up.

// New types in palace.rs
pub struct ActivityEvent {
    pub state: ActivityState,
    pub detail: Option<String>,
    pub timestamp: chrono::DateTime<chrono::Utc>,
}

pub enum ActivityState {
    Idle, Embedding, SidecarChecking, FoundRelevant,
    Extracting, Maintaining,
    ToolAction { action: &'static str },
}

// New on PalaceBuilder
pub fn activity_sink(self, sink: Arc<dyn Fn(ActivityEvent) + Send + Sync>) -> Self;

// New on Palace (pub)
pub activity_sink: Option<Arc<dyn Fn(ActivityEvent) + Send + Sync>>;

Motivation

Required for the jcode → mempalace Mode C library migration (mp-migration 5/8). jcode's MemoryEventSink = Arc<dyn Fn(ServerEvent) + Send + Sync> drives the TUI info widget's MemoryActivity snapshot, which has 4 pipeline states (search → verify → inject → maintain) and 24 event kinds. jcode publishes transitions at every meaningful point and the TUI reads them to render the "memory is searching…" / "found 3 relevant" / "extracting from session…" indicators.

mempalace has the same internal concept (in background.rs via tracing::info!) but it's:

• Not exposed on the public API
• Not typed — just log strings
• Not consumable by library users

This PR ships the type and the registration path. The emit-calls from the impl MemoryProvider for Palace methods (add_drawer, forget, search, extract_from_transcript, and the background.rs consolidation/lesson-decay loops) are scoped to a follow-up because they touch more lines.

This is PR 5/8 in the migration series. Independent of the trait-method PRs (1/2/4) — type and builder only.

Change

crates/core/src/palace.rs:

• pub struct ActivityEvent + pub enum ActivityState (~50 lines, includes the 6 state variants)
• Palace::activity_sink: Option<Arc<dyn Fn(ActivityEvent) + Send + Sync>> field
• Palace::emit_activity(state, detail) private helper that no-ops when no sink is set

crates/core/src/palace/builder.rs:

• PalaceBuilder::activity_sink: Option<Arc<dyn Fn(super::ActivityEvent) + Send + Sync>> field
• PalaceBuilder::activity_sink(sink) builder method (~15 lines)
• Wires the sink into Palace at open() time

Total: 93 lines added across 2 files.

Out of scope (deliberate follow-ups)

1. Wire Palace::emit_activity(Embedding, …) at the start of search and search_with_embedding.
2. Wire Palace::emit_activity(FoundRelevant { count }, …) after a non-empty result.
3. Wire Palace::emit_activity(ToolAction { action }, …) from the trait mutation methods (PR Fix real-run CLI flows and formatting issues #1 + fix: bug fixes to Rust core #2) where applicable.
4. Wire Palace::emit_activity(Extracting, …) from extract_from_transcript.
5. Wire Palace::emit_activity(Maintaining, …) from background.rs::run_consolidation and run_lesson_decay.

These are individual self.emit_activity(...) calls at the right pipeline points; each is a 1-3 line change. Tracking them as separate PRs keeps this one focused on the API surface.

Test plan

• cargo test -p mempalace-core --lib → 1127 passed, 0 failed (228s)
• cargo check -p mempalace-core clean
• cargo fmt --check -p mempalace-core clean
• CI green on ubuntu/macos/windows

Series status

#	Title	Status
6	docs(palace): document Embedder reuse on search_with_embedding	merged #16
7	feat(palace): add tags/trust/access_count/reinforcements/superseded_by to Drawer	open #17
8	feat(palace): add MemoryProvider::graph_stats_legacy (jcode shape)	open #18
1	feat(palace): add boost/decay/reinforce/supersede/set_metadata to MemoryProvider	open #19
2	feat(palace): add tag/untag/link/list_tags to MemoryProvider	open #20
4	feat(palace): add MemoryProvider::recent for retention-ranked recall	open #21
3	feat(palace): add MemoryScope::All and Wing/Room variants	open #22
5	feat(palace): add ActivityEvent sink on PalaceBuilder	this PR

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