improve: tangential velocity boundary condition at obstacle edges

[jayf0x]

Problem

Current branchless (1-obs) weight zeroes velocity inside obstacles but provides no explicit tangential (slip) condition at boundaries. A thin layer of incorrect velocity just outside text slightly distorts the flow path.

Proposed change

After the gradient-subtract pass, add a boundary-condition step:

1. Compute obstacle gradient ∇obs via 2-tap finite difference of the obstacle texture — this gives the surface normal.
2. For cells where obs > 0 && obs < 0.5, subtract the normal component from velocity (project to tangential).

Acceptance criteria

• Boundary condition pass runs after gradient-subtract
• Fluid flows tangentially along obstacle edges rather than stagnating
• No observable performance regression at simResolution=0.5

Files likely touched: src/core/shaders.ts, src/core/wgsl-shaders.ts, src/core/simulation.ts