fix: peekLine left buffer in inconsistent state after compaction

peekLine captured csvBuffer.pos at entry and restored that absolute
value at exit. fetchData, however, can compact the buffer mid-scan
(shifting begin to 0 and adjusting pos). The restored absolute
savedPos then pointed past the actual line start in the new layout.
Subsequent skipLine(line.length()) advanced from the wrong base,
eventually pushing pos out of buffer bounds and throwing
StringIndexOutOfBoundsException - or, more dangerously, silently
skipping records.

Restore csvBuffer.pos to csvBuffer.begin instead. The invariant at
peekLine entry is pos == begin (the caller just consumed previous
records via skipLine), so this is equivalent in the no-compact
case, and begin is itself shifted by compaction so it stays
correct across a fetchData compact.

JMH: FastCsvReadBenchmark unchanged within noise.

Author: osiegmar

lib/src/intTest/java/blackbox/reader/AbstractSkipLinesTest.java

@@ -248,6 +248,23 @@ void peekLineSeesContentBeyondInternalBuffer() {
                 .isStartingLineNumber(3).fields().containsExactly("value1", "value2"));
     }
 
+    @Test
+    void peekLineSurvivesBufferCompaction() {
+        // Force buffer compaction during peekLine's scan: skip a long first line
+        // (so begin advances past the internal readSize), then peek a second long line
+        // (so the scan exhausts the buffered tail and triggers compact-then-fetch).
+        // A bug that captures `pos` as an absolute index and restores it post-compact
+        // would leave the parser positioned mid-record afterwards.
+        final int chunk = 8500;
+        final String data = "S".repeat(chunk) + "\n" + "P".repeat(chunk) + "\nlast";
+
+        final CsvReader<CsvRecord> csv = crb.ofCsvRecord(new StringReader(data));
+
+        // Predicate matches the 3rd line. Forces peekLine on the 2nd (compaction-triggering)
+        // line to be skipped past, exercising the post-peek pos/begin invariants.
+        assertThat(csv.skipLines("last"::equals, 5)).isEqualTo(2);
+    }
+
     // Probe lengths around the 8 KiB lookahead boundary (8190..8194) with both LF and CRLF.
     // A predicate that matches only on the *exact* line content fails if peekLine
     // truncates at the boundary or splits CR/LF across a refill.

lib/src/main/java/de/siegmar/fastcsv/reader/StrictCsvParser.java

@@ -358,8 +358,6 @@ public String peekLine() throws IOException {
             throw new EOFException();
         }
 
-        final int savedPos = csvBuffer.pos;
-
         for (; csvBuffer.pos < csvBuffer.len || csvBuffer.fetchData(); csvBuffer.pos++) {
             final char c = csvBuffer.buf[csvBuffer.pos];
             if (c == CR || c == LF) {
@@ -368,7 +366,7 @@ public String peekLine() throws IOException {
         }
 
         final String s = new String(csvBuffer.buf, csvBuffer.begin, csvBuffer.pos - csvBuffer.begin);
-        csvBuffer.pos = savedPos;
+        csvBuffer.pos = csvBuffer.begin;
         return s;
     }