v0.55.0

dag gains the two operations its one consumer was hand-rolling beside it,
and the one-function registry package folds into uniq.

Added

• dag.TopoSort and dag.TopoSortFunc: Kahn topological sort with a
  deterministic tie-break, so the order depends on the input alone. On a
  cycle the result holds the nodes that could still be ordered and the
  error is dag.ErrCycle.
• dag.LongestPath: the number of nodes on the longest path from a start
  node, memoised, for critical-path scoring.
• uniq.Merge: base followed by extra with every key unique, or an error
  wrapping uniq.ErrDuplicate that names the repeated key.

Removed

• registry. Replace registry.MergeUnique(kind, builtins, user, slugOf, mark)
  with uniq.Merge(builtins, user, slugOf). Apply mark to a clone of
  builtins before the call and wrap the error with kind at the call site.