Fix flex-basis 0% collapse, fluid max-width centering, letter-spacing snap, video frame normalization

- Keep basis/h/min-h [0%] literal (0% content-sizes on an indefinite axis;
  0px is definite zero) and fold grow + zero-basis into flex-1 - a
  flex:1 1 0% child no longer collapses to height 0 in auto-height columns
- Width centering accepts per-viewport caps (max-width: min(Npx, fluid))
  with symmetric gaps -> mx-auto + banded max-w instead of literal banded
  margins that drift off-centre between captured widths; non-painted
  viewports no longer poison the inference
- letter-spacing keeps sub-0.1px precision (no zero snap); style gate
  normalizes Chromium's letter-spacing "normal" <-> 0px serialization
- Videos pause + seek to frame 0 before every source AND clone screenshot,
  removing playback-time nondeterminism from perceptual evidence

278 tests pass (19 new), typecheck clean.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

compiler/test/gates.test.ts

@@ -0,0 +1,38 @@
+import { describe, it } from "node:test";
+import assert from "node:assert/strict";
+import { letterSpacingEquivalent } from "../src/validate/gates.js";
+
+// Chromium serializes a computed `letter-spacing: 0` back as the keyword `normal`. The emitter, after
+// snapping a sub-0.1px authored tracking to 0, ships `letter-spacing: 0px` — which the CLONE then
+// reports as `normal` too, but a source that authored an explicit near-zero px can land on either
+// spelling. Gate 4 must treat `normal` and `0px` as equal, or every such node fails on spelling alone.
+describe("gate4 letterSpacing normal ↔ 0px normalization", () => {
+  it("treats `normal` and `0px` as equivalent", () => {
+    assert.ok(letterSpacingEquivalent("normal", "0px"));
+    assert.ok(letterSpacingEquivalent("0px", "normal"));
+  });
+
+  it("treats `normal` and `normal` as equivalent", () => {
+    assert.ok(letterSpacingEquivalent("normal", "normal"));
+  });
+
+  it("treats a sub-2px authored tracking vs `normal` as equivalent (within ±2px)", () => {
+    // source computed `-0.08px`, clone serialized `normal` — a 0.08px delta, well within tolerance.
+    assert.ok(letterSpacingEquivalent("-0.08px", "normal"));
+    assert.ok(letterSpacingEquivalent("normal", "-0.0375px"));
+  });
+
+  it("still equates two close real px values (−0.24px vs −0.2px)", () => {
+    assert.ok(letterSpacingEquivalent("-0.24px", "-0.2px"));
+  });
+
+  it("still FAILS a genuine tracking difference beyond ±2px", () => {
+    assert.ok(!letterSpacingEquivalent("4px", "normal"));
+    assert.ok(!letterSpacingEquivalent("normal", "-3px"));
+    assert.ok(!letterSpacingEquivalent("6px", "2px"));
+  });
+
+  it("passes when the source did not constrain the property (undefined)", () => {
+    assert.ok(letterSpacingEquivalent(undefined, "0px"));
+  });
+});