fix: correct RCH topic subscriptions and diagnostics

[brothercorvo]

Summary

• Make the Rust topic subscriber map authoritative and validate normalized topic/subscriber invariants during snapshot restore.
• Add the v3 additive SQLite topic/subscriber index migration and cascade subscriber cleanup when topics are deleted.
• Route identity updates through the SDK import/activate/announce contract, preserve SDK errors, and expose topic/subscriber runtime diagnostics.
• Preserve the existing /Topic, /Subscriber, Python-shaped fields, snapshot, MessagePack, fan-out, and UI-facing contracts.

Verification

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace (all executed tests passed; one existing external-mesh test ignored)

UI files were not changed, so UI checks were not required.

[brothercorvo]

@codex review

[chatgpt-codex-connector]

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[chatgpt-codex-connector]

Allow compatibility subscribers without topics

When /Subscriber or /Subscriber/Add creates a record with an omitted or unknown TopicID—which create_subscriber_record still permits via unwrap_or_default() without checking state.topics—the subscriber is persisted and later included by load_r3akt_read_snapshot; this new rejection then makes with_r3akt_core and full-snapshot r3akt_command requests return 500. The same failure affects pre-v3 databases containing orphan subscribers left by the previous topic-deletion behavior. Repair or exclude these compatibility rows during migration/snapshot construction, or validate them at the northbound route before enforcing this invariant.

AGENTS.md reference: AGENTS.md:L132-L137

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