fix: split the bloom blur so every toolchain can type-check it

[oddurs]

Problem

The first CI run on main failed to build:

Sources/Windflow/Canvas.swift:449:9: error: the compiler is unable to
type-check this expression in reasonable time

blurBloom inlined the index arithmetic for both blur passes into two nested
withUnsafeMutableBufferPointer closures. Swift 6.3.1 locally handles it; the
toolchain on the macos-15 runner gives up. Green locally, red in CI — the
worst failure mode for a build only one machine ever compiles.

Approach

The two passes are now blurRows and blurColumns: static methods with
explicit signatures, with the incoming and outgoing indices of the sliding
window as named locals rather than sub-expressions. Nothing left is large enough
to strain inference. The loop structure and the arithmetic are unchanged.

Look at this sceptically

• Is it really behaviour-preserving? The composited frame is byte-identical
  before and after at a fixed seed (WINDFLOW_SEED=99, 800×450, t=20s). The
  paint checksum alone would not have proved this — bloom never touches the
  paint buffer — so the comparison is of the final PNG.
• This class of bug can recur. Nothing stops the next deeply nested
  expression from tripping a different toolchain, and CI is the only thing that
  would catch it. The composite loop in the same file nests nine closures and is
  the obvious next candidate.