[bfrancojr]

Fixes #161

A refused dataset row now names its own line in the stream, as protowire-go does. Before, every row was parsed as if it started at 1:1, so in a long stream the error said 1:3 whatever row it came from.

The same cause reached further than the issue's error case. A row's pos and its cells' positions were also counted from the row, and so was the position of an error from Scan. All of them now match Go.

What changed

• The reader keeps the stream position of the start of its buffer and moves it past every byte it consumes, as Go's base and posAt do.
• Each row is parsed by a lexer that starts at the row's position in the stream, as Go's newParserAt does.
• pxf::Lexer gains a constructor that takes that start position (line, column, offset). This adds to the public API. The existing constructor is unchanged.

Evidence

A Go probe run against protowire-go 903b4a7, with MaxBytesLiteralLength: 191, gives these lines. The new test DatasetReaderPositions.MatchProtowireGo expects them verbatim:

Case	protowire-go output (= C++ now)	C++ before
first row	2:3: bytes literal decodes to more than …=191 bytes	1:3: …
later row (after 2 good rows and a blank line, indented)	rows pos=2:1@23, 3:1@29; error 5:5: …	1:1@0, 1:1@0; 1:3: …
rows on the header's line	row pos=1:24@23, cell 1:26@25; error 1:34: …	1:1@0, 1:3@2; 1:3: …
a row's second line	2:5: …	2:5: … (matched already: that row starts on line 1)
arity	3:4: @dataset row has 1 cells, expected 2 (column count)	1:1: …
bad token	3:7: expected ',' or ')' in @dataset row, got integer	1:5: …
row 3001, about 18,000 bytes in (several 4 KiB reads)	3002:3: @dataset row has 2 cells, expected 1 (column count)	not run before the fix
Scan, a column the type lacks	3:3: @dataset column "nope": google.protobuf.FieldDescriptorProto has no field "nope"	not run before the fix
Scan, a cell of the wrong type	3:3: column number: expected integer for field "number"	1:1: …

With the fix stashed, 8 of the 9 cases fail. The exception is "a row's second line", for the reason in the table. With the fix, ctest passes 729/729 locally (macOS, Homebrew protobuf).

Classification

• The change can be undone with a revert.
• It adds one public constructor and changes no existing signature or type.
• Callers see one difference: a row's positions are now counted from the start of the stream, not from the start of the row. That makes C++ match Go, which is the reference port.
• It was measured against Go on the inputs above.

Caveat

This machine has clang-format 23, while the CI job runs 18. The new lines are clean under 23, and the CI clang-format job will check them under 18.