pxf: TableReader streaming + bind_row per-row binding

[trendvidia]

Summary

Second of three PRs in the Python v0.72–v0.75 catch-up. Wraps `protowire-cpp`'s `TableReader` (cpp #13) so consumers can stream `@table` rows from an in-memory buffer instead of materializing the whole row sequence up front.

Python surface (in `src/protowire/pxf.py`):

```python
reader = pxf.TableReader.from_bytes(data)
print(reader.type, reader.columns)
for row in reader:
msg = TradeMsg()
pxf.bind_row(msg, reader.columns, row)
handle(msg)

Optional: chain a second @table from the same input

tr2 = pxf.TableReader.from_bytes(reader.tail())
```

• `from_bytes(data)` accepts `bytes` or `str`. (A file-like / chunked-IO bridge is a possible follow-up.)
• `type` / `columns` / `directives` properties expose the parsed header.
• Standard iterator protocol + a `next_or_none()` non-raising variant.
• `tail()` returns the unconsumed buffer for multi-`@table` chaining.
• `scan(msg)` is `Next` + `bind_row` in one call.

`pxf.bind_row(msg, columns, row, *, skip_validate=True)` is exported as a free function for callers iterating `Result.tables[i].rows` from the materializing path. Strategy is format-and-reparse — matches the cpp port's `BindRow`. Cells render as a synthetic PXF body and run through `unmarshal`, reusing every branch of the existing decoder (WKT timestamps / durations, wrapper-nullability, enum-by-name, `pxf.required` / `pxf.default`, oneof).

FFI (`src/_protowire/module.cc`):

• `PyTableReader` class wraps `protowire::pxf::TableReader`. The wrapped `istringstream` is held alongside the reader so its lifetime is bound to the Python object (cpp `TableReader` takes a non-owning `std::istream*`).
• Cells marshal through the same `CellToPyTuple` helper introduced in PR pxf: cpp pin to v0.75.0 + Result.directives/tables + validate_descriptor #6.
• `iter` returns self; `next` raises `StopIteration` at EOF.

Test plan

• 22 new tests in `tests/test_pxf_table_reader.py` covering header parsing (happy path, str input, no-@table, empty input, leading directives, 64 KiB cap), iteration (ordered rows, zero-rows-stops, next_or_none EOF, three-state cells, sticky arity, parens-in-strings, comments between rows), `tail` chaining, `bind_row` + `scan` (binding by name, absent-leaves-default, null-clears-wrapper, bytes cell round-trip, mismatched-columns / unknown-column errors, string escape)
• All 84 tests pass locally (62 on main → +22 new)
• CI green (Linux / macOS / Windows × Python 3.10–3.13, codeql)