docs(pdftract-4ogx4): add verification note for char_validity_rate signal evaluator

The LowCharValiditySignal and HighCharValiditySignal evaluators were already
implemented in classify.rs. All acceptance criteria are met:
- rate < 0.4 → BrokenVector with strength 0.80
- rate > 0.85 → Vector with strength 0.90
- middle band (0.4-0.85) → None
- no text → None

All 80 classification tests pass.

notes/pdftract-4ogx4.md

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+# pdftract-4ogx4: Signal evaluator char_validity_rate
+
+## Status: PASS
+
+This bead requested implementing two signal evaluators for `char_validity_rate`:
+- `rate < 0.4` → BrokenVector with strength 0.80
+- `rate > 0.85` → Vector with strength 0.90
+
+## Implementation Location
+
+File: `crates/pdftract-core/src/classify.rs`
+
+### LowCharValiditySignal (lines 222-240)
+```rust
+struct LowCharValiditySignal;
+
+impl SignalEvaluator for LowCharValiditySignal {
+    fn evaluate(&self, ctx: &PageContext) -> Option<Vote> {
+        if ctx.has_text() {
+            let validity = ctx.char_validity_rate();
+            if validity < 0.4 {
+                return Some(Vote::broken_vector(0.80));
+            }
+        }
+        None
+    }
+}
+```
+
+### HighCharValiditySignal (lines 242-260)
+```rust
+struct HighCharValiditySignal;
+
+impl SignalEvaluator for HighCharValiditySignal {
+    fn evaluate(&self, ctx: &PageContext) -> Option<Vote> {
+        if ctx.has_text() {
+            let validity = ctx.char_validity_rate();
+            if validity > 0.85 {
+                return Some(Vote::vector(0.90));
+            }
+        }
+        None
+    }
+}
+```
+
+## Acceptance Criteria Verification
+
+| Criterion | Status | Evidence |
+|-----------|--------|----------|
+| `rate = 0.3 → Some(Signal { 0.80, BrokenVector })` | PASS | `LowCharValiditySignal` returns `Vote::broken_vector(0.80)` for `validity < 0.4` |
+| `rate = 0.9 → Some(Signal { 0.90, Vector })` | PASS | `HighCharValiditySignal` returns `Vote::vector(0.90)` for `validity > 0.85` |
+| `rate = 0.6 → None` (middle band) | PASS | Both signals return None for rates between 0.4 and 0.85 (inclusive) |
+| `rate = None (Phase 4.7 not run) → None` | PASS | Both signals check `ctx.has_text()` first, returning None if no text |
+
+## Test Coverage
+
+All classification tests pass (80/80):
+
+- `test_page_classifier_low_char_validity` - Tests 20% validity → BrokenVector
+- `test_page_classifier_vector_pure_text` - Tests 97% validity → Vector  
+- `test_page_classifier_default_vector` - Tests 70% validity (middle band) → default Vector classification
+- `test_page_classifier_confidence_in_range` - Tests various validity rates
+
+## Integration
+
+Both signal evaluators are registered in `PageClassifier::new()` (lines 310-320):
+```rust
+signals: vec![
+    Box::new(NoTextOperatorsSignal),
+    Box::new(InvisibleTextWithImageSignal),
+    Box::new(HighImageCoverageSignal),
+    Box::new(LowCharValiditySignal),        // ← rate < 0.4 → BrokenVector
+    Box::new(LowDensitySignal),
+    Box::new(HighCharValiditySignal),       // ← rate > 0.85 → Vector
+],
+```
+
+The signals are evaluated in order after the Hybrid evaluator check. Low char validity fires before high validity, ensuring that broken encodings are correctly identified even when high validity signals might also fire.
+
+## Conclusion
+
+The bead was already fully implemented. No code changes were required. All acceptance criteria are met and tested.