feat(v7-q09): per-brick palette help icons with inputs/outputs/norm popups

Adds a ? help icon next to every tile in the Node Library palette
(both bricks and adapters). Each popup explains: what the brick is,
why you'd reach for it, expected inputs (shape + params), expected
outputs (shape), and where normalization is needed (pre/post-norm +
which adapter to drop).

Schema:
- HelpTopic interface gains optional inputs / outputs / normalization
  fields.
- HelpModal renders three new Sections when those fields are present.

Coverage:
- HELP_TOPICS extended with 25 brick_<kind> entries: attention,
  gated_attention, mla, mla_absorb, mistral4_mla, dsv4_attention,
  bailing_mla, gqa_sliding, cca_attention, gemma4_drafter,
  nemotron_h_mtp, bailing_linear, gdn, kda, mamba3, mlstm, moe,
  bailing_moe, nsa, lightning_indexer, csa_hca, engram, mlp,
  abs_pos_embed, per_layer_embed.
- And 6 adapter_<kind> entries: merge_heads, split_heads,
  transpose_bnsd, linear_bridge, rmsnorm, residual.

UI wiring:
- Palette.tsx renders <HelpIcon topic="brick_<kind>"> on each tile.
- The ? icon wrapper stops drag/mouse/pointer-down propagation so
  opening the popup doesn't kick off a brick drag.

Tests (5/5 PASS, vbgui/tests/PaletteHelpIcons.test.tsx):
- every brick tile has a help-icon-brick_<kind> testid
- every adapter tile has a help-icon-adapter_<kind> testid
- HELP_TOPICS schema is complete for both arrays + brick entries
  carry inputs/outputs/normalization fields
- clicking brick_attention ? opens modal with (B, S, H) + RMSNorm
  guidance
- clicking adapter_rmsnorm ? opens its modal

Regression: vbgui vitest 451 -> 474 (+23) PASS.

Closes V7-Q09 (cppmega-mlx-g9ky).

Author: apstenku123