[A-followup] Use hebb SimEngine::apply_seed instead of hand-rolled in-memory loop

[ammar-s847]

Follow-up from the PR review on 2026-05-27.

Context: The apply_seed tool (#17 / PR #52) hand-rolls the folder-free seeding path in core/src/engine/mod.rs (EngineCommand::ApplySeed loops add_neuron_with_kind + add_edge directly) because Stream B did not wait for the library method. Meanwhile hebb#12 (G1) adds a clean, validated, roll-back-capable SimEngine::apply_seed(&Seed) -> SimSeedReport to the hebb crate. Right now that library method is unused by the app — duplicated logic.

MVP: the app uses the library method; the hand-rolled loop is removed.

Acceptance:

• hebb#12 merged and a new hebb version published to crates.io
• hebb_app core/Cargo.toml pin bumped to that version
• EngineCommand::ApplySeed (no-folder branch) calls SimEngine::apply_seed instead of the manual loop
• EngineSeedReport maps from SimSeedReport
• existing apply_seed tests still green (incl. the 200-node small-world test)

Dependencies: hebb#12 (merge + publish).

Nice-to-haves: surface the library validation/rollback errors as ToolError::Substrate so a bad seed reports cleanly to the agent.