fix(cli): enrich template cardinality error with per-candidate presence and profile hints

[wpfleger96]

Problem

buzz channels create --template fails with a bare-pubkey duplicate-instance error when a persona has more than one live instance. When duplicate instances share the same display name and avatar, the error gives the operator no signal to identify which instance is stale.

What changed

crates/buzz-cli/src/commands/channels.rs (boundary) + one visibility line in crates/buzz-cli/src/commands/users.rs (fn presence_subject → pub(crate)).

Hint fetch architecture

assemble_roster_resolution<F, Fut> — post-fetch stage extracted from build_roster_resolution as an injectable-fetcher async generic. It contains: archive-filter-based duplicate detection, the conditional fetch_hints call, and finalize_roster_resolution delegation. Accepting the fetcher as a closure makes the wiring directly testable without a relay.

build_roster_resolution reduces to: gather slugs → tokio::join!(scan, archive) → delegate with the real fetch_candidate_hints closure.

Zero hint queries on the happy path: assemble_roster_resolution only invokes the fetcher when duplicate live instances exist after archive filtering. Untrusted archive snapshot (Err) conservatively treats all found instances as live.

When duplicates exist, both queries (presence kind:40902 + profile kind:0) run concurrently via tokio::join! inside a single 3-second tokio::time::timeout. On expiry, bare pubkeys are printed promptly.

Response-to-map conversion — build_hint_map

Sync production function taking raw presence/profile event slices. Relay-signed presence events carry the agent pubkey in the p tag (not the event author); build_hint_map calls presence_subject from users.rs (now pub(crate)) to resolve the correct key.

Hint display — format_candidate

Appends [online/offline, profile updated YYYY-MM-DD] (or subset) to each candidate pubkey in the error. profile_updated_at names the field correctly: kind:0 is replaceable state that desktop republishes on rename and profile reconciliation, so created_at reflects the last profile update, not provisioning time. Date formatted via chrono::DateTime::from_timestamp; out-of-range timestamps omit the date. Missing entries fall back to bare pubkeys.

Cardinality rule

apply_cardinality_rule remains pure and relay-free; zero/one/many semantics unchanged.

Tests

16 new tests in channels.rs:

assemble_roster_resolution wiring tests (4, #[tokio::test]):

• assemble_roster_resolution_duplicate_pair_invokes_fetcher_with_their_pubkeys — fetcher called, error contains both pubkeys
• assemble_roster_resolution_single_instance_never_invokes_fetcher — panic closure proves fetcher not called
• assemble_roster_resolution_trusted_archive_removes_duplicate_suppresses_fetcher — archived instance resolves pair; fetcher suppressed
• assemble_roster_resolution_untrusted_archive_invokes_fetcher_conservatively — Err archive → fetcher called

build_hint_map boundary tests (5):

• build_hint_map_uses_p_tag_over_author_for_presence — relay author is not the map key; agent p-tag wins
• build_hint_map_presence_failure_profile_survives — one lookup failure does not suppress the other
• build_hint_map_profile_failure_presence_survives — symmetric
• build_hint_map_malformed_entries_are_skipped — no panics on malformed JSON
• build_hint_map_both_failures_yield_empty_map — total failure → empty map → bare pubkeys

format_candidate formatting tests (5):

• No hint → bare pubkey
• Presence only → [status]
• Profile updated only → [profile updated YYYY-MM-DD]
• Both → [status, profile updated YYYY-MM-DD]
• Both fields None → bare pubkey

apply_cardinality_rule integration tests (2):

• Hints present → decorated error
• Hint missing for one candidate → bare pubkey fallback for that candidate

Mutation check results:

• (a) Replace conditional fetch with HashMap::new(): 2 wiring tests fail
• (b) Remove emptiness gate (always fetch): 2 wiring tests fail
• Gut build_hint_map: 3 tests fail
• Delete p-tag selection: 1 test fails

337/337 buzz-cli tests pass. cargo fmt + cargo clippy clean.