skill: drop 4 redundant typography rules + add EMPIRICAL_VALIDATION

The v2.1 ablation sweep (n=10 × 4 brand niches × 3 providers, anchored to
commit 54c3a502, ~544 cells) confirmed these four rules carry no weight in
the skill:

- skill-typo-no-all-caps-body — duplicate of brand-ban-all-caps-body; brand
  version is more specific (reserves caps for labels + headings)
- skill-typo-codex-hero-ceiling-repeat — the codex-block restatement of
  skill-typo-hero-ceiling didn't add reinforcement on top of the universal
  rule
- skill-typo-scale-ratio — duplicate of brand-typo-modular-scale; same
  signal, brand version carries the clamp() / fluid implementation detail
- skill-typo-font-count — models don't reach for ≥4 font families in any
  niche we test, so the rule has no measurable effect

Each deletion is the Agent A / B / C / D Phase-2 audit recommendation;
none of the four ever validated under either prose state.

Adds EMPIRICAL_VALIDATION.md naming the seven cross-provider winners as the
trustworthy core, and documents the systemic findings (self-priming, detector
saturation, vocabulary anchoring) so future skill edits can avoid the same
traps.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

plugin/skills/impeccable/SKILL.md

@@ -35,10 +35,7 @@ Produce ready-to-ship, production-grade code, not prototypes or starting points.
 #### Typography
 
 - Cap body line length at 65–75ch.
-- Hierarchy through scale + weight contrast (≥1.25 ratio between steps). Avoid flat scales.
-- Cap font-family count at 3 (display + body + optional mono). More than 3 reads as indecision, not richness. One well-tuned family with weight contrast usually beats three competing typefaces.
 - Don't pair fonts that are similar but not identical (two geometric sans-serifs, two humanist sans-serifs). Pair on a contrast axis (serif + sans, geometric + humanist) or use one family in multiple weights.
-- No all-caps body copy. Sentences in ALL CAPS are unreadable at body sizes.
 - Hero / display heading ceiling: clamp() max ≤ 6rem (~96px). Above that the page is shouting, not designing.
 - Display heading letter-spacing floor: ≥ -0.04em. Anything tighter and letters touch; cramped, not "designed".
 - Use `text-wrap: balance` on h1–h3 for even line lengths; `text-wrap: pretty` on long prose to reduce orphans.