[0.4.0] validate OSW input and output, preserve zero-length edges and co-located nodes, remove warnings - #57

[sujata-m]

PR summary

Adds validation at both ends of conversion, fixes how degenerate and co-located geometry is carried between formats, and removes the warning subsystem.

Validation

• OSW input is validated with python-osw-validation before OSW→OSM starts. Rejected input returns status=False with the validator's per-feature issues, each naming the file and feature index; repeated messages are collapsed.
• OSW output is validated before OSM→OSW reports success, using the same settings that produced it.
• OSM input is rejected when any node coordinate carries more decimal places than coordinate_precision, for both XML and PBF. No tolerance: one digit over the limit is rejected. Coordinates are read as exact decimal strings, never floats.
• coordinate_precision and allow_zero_length_lines are passed to the validator, so input, output and conversion are judged by one set of rules.

Error messages

• A corrupt OSM file returns invalid input file, the OSM file is corrupted and could not be read. The problem is at line N, column M. instead of printing a traceback and surfacing raw parser text. Reads identically whether the failure surfaces during validation or during conversion.
• Unreadable OSW archives (missing, not a zip, no .geojson inside) are reported in plain language rather than OS error numbers, and echo only the file name — previously the caller's absolute path leaked into the message.

Geometry

• Zero-length edges survive both directions when allow_zero_length_lines is set: OSW e1 = (n0, n0) becomes OSM w1 = [n0, n0], and back again. With the setting off they collapse to a single point. Required two fixes — restoring the nd ref ogr2osm drops from a single-node way, and a self-loop guard in OSMGraph.simplify() that was splicing the node into its own way and deleting both.
• Co-located points are no longer merged. Two OSW point features at one coordinate stay two OSM nodes even with identical tags. Way vertices still merge onto node features, so edges stay connected and kerb nodes stay attached.
• Edge endpoints are restored from _u_id/_v_id. ogr2osm resolves way vertices by coordinate, so an edge between two co-located nodes collapsed onto whichever node was created first. The references are recorded while features are read and applied before renumbering.

Breaking changes

• Response.warnings and helpers/warnings.py are removed. Callers reading response.warnings will get an AttributeError. Coordinate precision is now enforced at the input gate rather than reported after the fact.
• allow_zero_length_lines now defaults to True, matching the validator default. Set it to False for the previous behaviour.

New config: validate_input and validate_output, both defaulting to True, each with a Formatter(...) kwarg to opt out.

Testing

311 tests pass. Every OSM fixture in the repo was run through conversion and its output validated before enabling output validation by default — all compliant, so no existing test needed changing.