Adds --explain-slots flag to vera check. After a successful type-check, prints a slot resolution table per function showing which parameter position each @T.n index refers to.

$ vera check examples/math.vera --explain-slots
OK: examples/math.vera

Slot environments (index 0 = last occurrence in signature):

  fn divide(@Int, @Int -> @Int)
    @Int.0  parameter 2 (last @Int)
    @Int.1  parameter 1 (first @Int)

Motivation: The dominant VeraBench failure mode is De Bruijn slot ordering (9pp gap vs Python). This flag lets agents self-correct via the check → read table → fix loop without any language change. Also useful for human reviewers.

Scope:

• New --explain-slots flag on vera check (and --json mode extends output with slot_environments array)
• New vera/slots.py utility module
• Prescriptive documentation updates to SKILL.md, DE_BRUIJN.md, CLAUDE.md, README.md, and spec ch03

Closes #183 (won't fix — inline annotations are contraindicated; see that issue for the design rationale).