boundarySlice.ts has regrown to 1000 lines (5 concerns) — past the threshold that produced it from modelStore.ts split (#202)

[mkofler96]

What's wrong

web/src/store/boundarySlice.ts grew from 549 → 1000 lines (+82%) across the four most recent PRs (#417-mkofler96/KoFEM#420: explicit tie connections, surface-to-point couplings, shell/solid MPC validation, surface-load overlap fix). It now mixes five distinct concerns in one file:

1. Tie-group state/actions
2. Coupling-group state/actions, including reference-point node lifecycle
3. Original BC/load group state/actions
4. rebuildLoads / rebuildSurfaceLoads
5. Viewport pick-session state (tie-side toggling, coupling draft)

Why it matters

boundarySlice.ts exists because issue #202 ("modelStore.ts is a 988-line monolith — split into focused state slices," closed) split it out as one of several focused slices. It has now grown back past that same ~1000-line threshold that made #202 worth fixing in the first place — the split bought headroom, not a permanent fix, and each new coupling feature has been landing in the same file.

Not a duplicate of the other outstanding "file has outgrown itself" issues: #401 is lib/shellize.ts, #371 is solver.worker.ts, #364 is BoundaryConditionLayer.tsx — none target boundarySlice.ts.

Suggested fix

Split along the same lines #202 used: tie-group and coupling-group state/actions are good candidates to move into their own slices (tieSlice.ts, couplingSlice.ts), leaving boundarySlice.ts with the original BC/load concerns plus the shared rebuild logic.

Found during a scheduled codebase health scan (fresh review of commits 57e0346..HEAD).