docs(pdftract-cbrbg): add verification note

notes/pdftract-cbrbg.md

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+# Verification Note: pdftract-cbrbg
+
+## Summary
+Implemented span flag detector for Phase 4.1 as specified in the plan.
+
+## Implementation Details
+
+### Files Modified
+- `crates/pdftract-core/src/lib.rs` - Added span_flags module
+- `crates/pdftract-core/src/span_flags.rs` - New module (467 lines)
+
+### Functionality
+Implemented `detect_span_flags()` function that returns a u8 bitmask combining 5 style flag bits:
+- `flags::BOLD` (bit 0)
+- `flags::ITALIC` (bit 1)
+- `flags::SMALLCAPS` (bit 2)
+- `flags::SUBSCRIPT` (bit 3)
+- `flags::SUPERSCRIPT` (bit 4)
+
+### Detection Logic (per plan lines 1667-1671)
+
+**BOLD detection** (set if ANY of):
+- Font name contains "Bold", "Bd", "Black", "Heavy", "ExtraBold", etc.
+- FontDescriptor /Flags bit 18 (ForceBold) set
+- FontDescriptor /StemV > 120
+
+**ITALIC detection** (set if ANY of):
+- Font name contains "Italic" or "Oblique"
+- FontDescriptor /ItalicAngle != 0
+
+**SMALLCAPS detection** (set if ANY of):
+- Font name contains "SC", "SmallCaps", or ".sc"
+- FontDescriptor /Flags bit 3 (Symbolic/SmallCap) set
+
+**SUBSCRIPT detection**:
+- `text_rise < -0.1 * font_size`
+
+**SUPERSCRIPT detection**:
+- `text_rise > 0.1 * font_size`
+
+### API
+```rust
+use pdftract_core::span_flags::{detect_span_flags, FontInfo, flags};
+
+let font = FontInfo::new()
+    .with_name("Times-Bold".to_string())
+    .with_flags(0x40000) // ForceBold bit
+    .with_stem_v(150.0);
+
+let span_flags = detect_span_flags(&font, -2.0, 12.0);
+assert!(span_flags & flags::BOLD != 0);
+assert!(span_flags & flags::SUBSCRIPT != 0);
+```
+
+## Acceptance Criteria Status
+
+### PASS
+- [x] "Times-Bold" font → BOLD set
+- [x] "Helvetica-Italic" font → ITALIC set
+- [x] "Times-BoldItalic" font → BOLD | ITALIC set
+- [x] text_rise -2pt with font_size 12pt → SUBSCRIPT set (rise/size = -0.167 < -0.1)
+- [x] text_rise +1.5pt with font_size 12pt → SUPERSCRIPT set
+- [x] text_rise -0.5pt with font_size 12pt → NEITHER (rise/size = -0.042, within threshold)
+- [x] font.descriptor.flags bit 18 set → BOLD set
+- [x] /StemV 150 → BOLD set
+
+### Additional Test Coverage
+- [x] Subset prefix handling (ABCDEF+Times-Bold)
+- [x] Small caps with ".sc" suffix pattern
+- [x] ItalicAngle detection (non-zero angle)
+- [x] All flags combination (BoldItalic + SmallCaps + Superscript)
+- [x] Zero font_size edge case (no division by zero)
+- [x] Mutually exclusive SUB/SUPER (can't have both)
+
+## Test Results
+```
+cargo test -p pdftract-core span_flags --lib
+test result: ok. 27 passed; 0 failed; 0 ignored; 0 measured
+```
+
+## Quality Gates
+- [x] `cargo check --all-targets` - PASSED
+- [x] `cargo clippy --all-targets -- -D warnings` - PASSED
+- [x] `cargo fmt` - APPLIED
+- [x] All tests pass
+
+## Git Commit
+- Commit: `cad7d2c`
+- Message: `feat(pdftract-cbrbg): implement span flag detector for Phase 4.1`
+- Pushed to: `github/main`
+
+## References
+- Plan: Phase 4.1 Flag detection (lines 1648-1653, 1667-1671)
+- Bead: pdftract-cbrbg