feat: you can now supply your own traversal

[DoodleBears]

Why

The Traversal layer has always been one function of lattice position — the seam the whole
layered design leaves open. It was reachable only by writing your own renderer. Now it is an
option.

// a chevron that speeds up toward its tip
const chevron = (col, row, cols, rows) => {
  const mid = (rows - 1) / 2;
  const arm = Math.abs(row - mid) / (mid || 1);
  const v = arm * 0.6 + (col / (cols - 1 || 1)) * 0.4;
  return v * v;
};
createDotMatrix(el, { cols: 9, rows: 9, preset: "morph", order: chevron });

Change outline

It is free in both renderers — the CSS host derives each cell's animation-delay from
the same traversal value, so a custom traversal costs zero per-frame JS there too.

That asymmetry is the design: a custom envelope would not be free, because the CSS
animation is one shared @keyframes published per preset. So: traversal only, with a test
asserting the CSS host's --botui-o matches. A traversal the engine forgot to apply there
would produce two renderers that disagree — the one thing this library is arranged to make
impossible.

"Accelerating arrows" turned out not to need a new preset at all — only a traversal that
eases along its own axis. A centre point for the concentric orders is the same shape of
answer: geometry the traversal computes, so order covers it without a new option.

Special things to note

• Reversibility: additive; nothing existing changes. Review focus is the type work below.
• A type trap. order must be omitted from ResolvedDotMatrixOptions's Required
  alongside stagger — left in, the resolved type demands a traversal nobody supplied and
  DEFAULTS would need a default traversal, which is one more thing silently overriding the
  caller. OrderFn's fifth argument stays optional, so an existing four-argument traversal
  is still valid and one with no sense of direction is not forced to accept it.
• The README example is executed by a test. A snippet that does not compile is worse
  than none — it is the first thing a reader runs. The test runs it verbatim, on 1×1 / 1×7 /
  7×1 grids where a pasted traversal divides by zero, and asserts it really accelerates:
  an easing that did nothing would still run and still look plausible.
• It is a function, so it is not copyable, and the copy button omits it rather than
  emitting a prop that would not compile. Stated in the README and the option's docs, since
  it is a real constraint of this route.
• 260 tests (was 249), oxlint 0, registry ok, both changelog ledgers.

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📒 Files selected for processing (7)

• CHANGELOG.md
• CHANGELOG.zh.md
• packages/core/README.md
• packages/core/src/dot-matrix/field.ts
• packages/core/src/types.ts
• packages/core/test/custom-order.test.ts
• packages/core/test/readme.test.ts

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