docs(pdftract-342k4): add verification note for XFA detection

The detect_xfa function was already implemented in the codebase at the
time of bead assignment. This note documents the verification of the
existing implementation against the bead's acceptance criteria.

All 6 tests pass, covering all acceptance criteria:
- XFA stream presence → true
- XFA array packet form → true
- No XFA key → false
- XFA null → false
- No AcroForm → false
- XFA as indirect reference → true

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

notes/pdftract-342k4.md

@@ -0,0 +1,64 @@
+# pdftract-342k4: XFA Detection — Verification Note
+
+## Summary
+
+XFA detection (`detect_xfa`) was **already implemented** in the codebase at the time of bead assignment. No implementation work was required.
+
+## Implementation Status
+
+### Function Location
+- **File:** `crates/pdftract-core/src/forms/mod.rs`
+- **Lines:** 66-72
+- **Export:** Public API in `lib.rs` line 73
+
+### Implementation
+```rust
+pub fn detect_xfa(acroform: &Option<PdfDict>) -> bool {
+    acroform
+        .as_ref()
+        .and_then(|af| af.get("XFA"))
+        .map(|xfa| !matches!(xfa, PdfObject::Null))
+        .unwrap_or(false)
+}
+```
+
+This matches the bead's specification exactly.
+
+## Acceptance Criteria Verification
+
+All acceptance criteria PASS:
+
+| # | Criteria | Test | Status |
+|---|----------|------|--------|
+| 1 | PDF with AcroForm /XFA stream → true | `test_detect_xfa_xfa_stream` | ✅ PASS |
+| 2 | PDF with AcroForm /XFA [name, stream, ...] (packet form) → true | `test_detect_xfa_xfa_array_packet_form` | ✅ PASS |
+| 3 | PDF with AcroForm but no /XFA key → false | `test_detect_xfa_no_xfa_key` | ✅ PASS |
+| 4 | PDF with AcroForm /XFA null → false | `test_detect_xfa_xfa_null` | ✅ PASS |
+| 5 | PDF with no AcroForm → false | `test_detect_xfa_no_acroform` | ✅ PASS |
+
+**Additional coverage:** `test_detect_xfa_xfa_ref` covers indirect reference handling (also returns true).
+
+## Test Run
+
+```bash
+$ cargo nextest run -p pdftract-core --lib detect_xfa
+Starting 6 tests across 1 binary (2539 tests skipped)
+    PASS [   0.004s] (1/6) pdftract-core forms::tests::test_detect_xfa_no_acroform
+    PASS [   0.005s] (2/6) pdftract-core forms::tests::test_detect_xfa_no_xfa_key
+    PASS [   0.005s] (3/6) pdftract-core forms::tests::test_detect_xfa_xfa_stream
+    PASS [   0.005s] (4/6) pdftract-core forms::tests::test_detect_xfa_xfa_ref
+    PASS [   0.006s] (5/6) pdftract-core forms::tests::test_detect_xfa_xfa_null
+    PASS [   0.006s] (6/6) pdftract-core forms::tests::test_detect_xfa_xfa_array_packet_form
+────────────
+ Summary [   0.008s] 6 tests run: 6 passed, 2539 skipped
+```
+
+## Files
+
+- Implementation: `crates/pdftract-core/src/forms/mod.rs:66-72`
+- Tests: `crates/pdftract-core/src/forms/mod.rs:1437-1510`
+- Export: `crates/pdftract-core/src/lib.rs:73`
+
+## Conclusion
+
+**Bead complete.** The XFA detection function was already implemented with full test coverage. No code changes were required.