Modernize project structure

[treeform]

Summary

• update build and docs workflows to use actions/checkout@v5, treeform/setup-nim-action@v6, and Nimby
• install dependencies with cd .. && nimby install -g pixie/pixie.nimble so Nimby sees the checked-out Pixie package in workspace layout
• run CI tests on ubuntu-latest, windows-latest, and macos-latest with
  im r tests/tests.nim
• rename tests/all.nim to tests/tests.nim
• mark Pixie as Nim 2+ and update dependency floors to the currently checked-out package versions
• refresh README install/testing commands, badges, and .gitignore baseline

Dependency floors

• nim >= 2.0.0
• vmath >= 3.0.0
• chroma >= 1.0.0
• zippy >= 0.10.16
• flatty >= 0.3.4
• nimsimd >= 1.3.2
• bumpy >= 1.1.3
• crunchy >= 0.1.11

Verification

• local:
  im r tests/tests.nim
• local: git diff --check
• CI: ubuntu-latest, windows-latest, macos-latest passing