docs(meanderaw): 📝 amend the charter for branch loops and parallel branching

[JimmyPaolini]

Parent

#663

What to build

The charter is made honest about what the two families now draw, in the same pull request that
changes them, because its relaxation declaration is asserted in both directions — an invariant a
family does not relax must hold, and one it does relax must actually be broken.

Two corrections, and one architecture decision record explaining the trade.

• parallel now relaxes no-branching. Its relaxation row stops being empty, and the
  paragraph declaring that the empty row "is the whole point of the family rather than an
  omission" is reversed rather than quietly dropped.
• branch no longer closes no loop. Its row keeps no-branching; the prose claiming it
  "closes a loop at none" and the measured cycle count that distinguished it from negative are
  both corrected.

Acceptance criteria

• The relaxation declaration names no-branching for parallel, and the sweep passes in
  both directions
• The charter prose in both the project README and the agent guidance no longer claims
  branch is acyclic or that parallel relaxes nothing
• The measured cycle and junction counts quoted for both families match a fresh sweep
• An architecture decision record records the trade — branch acyclicity and parallel
  non-branching given up so that both families are identifiable by their interiors — with
  the measured collision counts as its evidence

Blocked by

• feat(meanderaw): ✨ draw both border rules in the branch family #666, refactor(meanderaw): ♻️ delete the comb modifier #667, feat(meanderaw): ✨ draw both border rules in the parallel family #668, and
  feat(meanderaw): ✨ drop the degenerate one-strand parallel combinations #669.