feat(palace): add tag/untag/link/list_tags to MemoryProvider (mp-migration 2/8)

[quangdang46]

Summary

Add 4 graph-mutation methods to MemoryProvider covering jcode's tag/link surface:

Method	Maps to jcode	Storage
tag(&id, tag)	MemoryManager::tag_memory	metadata["tags"] + metadata["tag:<n>"] = true
untag(&id, tag)	MemoryManager::untag_memory	removes from metadata["tags"] and metadata["tag:<n>"]
link(&from, &to, w)	MemoryManager::link_memories	metadata["links"] = Vec<{target, weight}>
list_tags()	closest: graph_stats.1	aggregates metadata["tags"] across drawers

Motivation

Required for the jcode → mempalace Mode C library migration (mp-migration 2/8). jcode's MemoryManager calls tag_memory and link_memories pervasively (memory.rs:1657, 1676) and the tool's tag and link actions (tool/memory.rs:328, 355) call them too. Without trait methods, the adapter would have to either re-implement or expose raw metadata writes.

This is PR 2/8 in the migration series. Includes a cherry-pick of PR #1 (boost/decay/reinforce/supersede/set_metadata) so this branch has all 9 mutation methods in one place. The two commits are independent in the series and can be reviewed/rebased independently.

Change

crates/core/src/palace.rs — adds 4 default methods to MemoryProvider trait (after set_metadata, before fingerprint):

async fn tag(&self, id: &DrawerId, tag: &str) -> anyhow::Result<()>
where Self: Sized;
async fn untag(&self, id: &DrawerId, tag: &str) -> anyhow::Result<()>
where Self: Sized;
async fn link(&self, from: &DrawerId, to: &DrawerId, weight: f32) -> anyhow::Result<()>
where Self: Sized;
async fn list_tags(&self) -> anyhow::Result<Vec<(String, usize)>>
where Self: Sized;

All four have the where Self: Sized bound required by the default-implementation helper.

The defaults use the metadata path so they don't depend on the KG being wired into Palace. The metadata shapes (Vec<String>, Vec<{target, weight}>) are designed to match the eventual KG triples:

• tag → kg.add_triple(subject: drawer_id, predicate: "has_tag", object: tag)
• link → kg.add_triple(subject: from, predicate: "relates_to", object: to, confidence: Some(weight))
• list_tags → kg.query_relationship(predicate: "has_tag") then group-by

Implementations that have a wired KG should override and use the KG API directly. The metadata path stays as a fallback for the jcode adapter's get-or-default usage.

Performance

• tag / untag / link: O(n) via the shared default_mutate_drawer helper (get_drawers + store.upsert).
• list_tags: O(n × m) where n is drawer count and m is avg tags per drawer. The KG-backed override can be O(1).

Both are fine for the embedvec tier (≤5 k drawers). Override for larger palaces.

Test plan

• cargo test -p mempalace-core --lib → 1127 passed, 0 failed (251s)
• 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	this PR
4	feat(palace): add MemoryProvider::recent for retention-ranked recall	next
3	feat(palace): add MemoryScope::All and Wing/Room variants	next
5	feat(palace): add ActivityEvent sink on PalaceBuilder	next

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