v0.52.3 — super-ux

• validate_front_matter_is_yaml parses all 30 shipped front-matter blocks with PyYAML
  (B-033): 7 SKILL.md, 15 commands, 8 .mdc mirrors. front_matter() reads a block line
  by line and never asks what a YAML parser would, so a : inside an unquoted scalar turns
  the whole block into an invalid mapping and ships anyway — green here, refused by every
  installer that parses YAML, and the hub copy freezes on the previous version. The family
  shipped exactly that twice in twelve days (sheleg-design 1.37.4 and 1.58.0, both
  mapping values are not allowed here), so the remedy is ported from the sibling that
  paid for it rather than reinvented.
• The two guards are the ones that history says matter. It fails closed when PyYAML
  is missing, because a guard that discloses and passes when its tool is absent is the hole
  the first fix left open; and it refuses an empty walk, because globs that all match
  nothing is a moved directory rather than a clean tree. Both watched: yaml blocked at
  import, and the three globs renamed.
• YAML_SELF_TEST is a permanent plant, not a one-off. This gate's whole claim is that
  it sees what front_matter() cannot; a parser that stopped refusing that shape would leave
  it green on a clean tree forever, so the shape is re-checked on every run.
• PyYAML installed in both workflows: setup-python puts a different interpreter on PATH
  than the runner's system one, which is where PyYAML is preinstalled.