Reduce duplication, dead code and oversized functions in the validation subsystem

[LukasOro]

Raised in review of #114 (#114 (comment)). Every item below re-verified against feat/validator-migration at b4e23ac, so the line numbers are current.

Duplication

• The to_rdf -> count -> from_rdf -> frame-or-compact sequence is written three times, in roundtrip.py, instance_checks.py and compliance.py. "application/n-quads" appears 9 times across those three modules (5 / 2 / 2). Extract a project_and_restore(...) helper.
• pipeline._check_variant (pipeline.py:353) is a compressed copy of pipeline._check_schema_jsonld (pipeline.py:260).

The third duplication from the same review comment, enforced_by = {v: k for k, v in rule_map().items()} written three times, is tracked in #125 together with the CLI/MCP filter-order divergence it belongs to.

Dead code

• resolve.dereference_and_bound() (resolve.py:495) has no caller anywhere in src/ or tests/; pipeline.py and mcp_server.py open-code the same sequence. Use it or delete it.
• 13 to_dict() definitions against 10 call sites in src/; the review counted seven with no caller.
• RoundtripResult.ok defaults to True (roundtrip.py:138) while InstanceRoundtripResult.ok defaults to False (instance_checks.py:43), and nothing reads either. Opposite defaults for the same concept is the kind of thing that becomes a bug the first time something does read it.

Naming and duplicated constants

• DEFAULT_MAX_DEPTH is 6 in resolve.py:45 and 40 in context_resolution.py:46. Same package, same name, different meaning and an order of magnitude apart. Rename both to say what they bound.
• "https://oo-ld.test/" is both loader.DEFAULT_HOST (loader.py:34) and compliance.RDF_BASE (compliance.py:48).
• SCHEMA_SUFFIX = ".schema.json" is defined in pipeline.py:40 and context_graph.py:27.
• Two UUID regexes: check_registry.py:55 and formats.py:78.
• Check.line() (report.py:71) duplicates the live renderer at cli.py:89-93 and has already drifted from it - column width 24 against 22. One of them is dead; the human output only goes through cli.py.

Length

C901 is in ruff's ignore list (pyproject.toml:287), so nothing flags these today:

Function	Lines
compliance._run_tests	126
pipeline._check_schema	92
pipeline._check_schema_jsonld	92
resolve.bound_schema	87
resolve.dereference	84

Splitting these is worth more than re-enabling C901, which would fail the build on code nobody is about to touch.

Constraint

All of it is behaviour-preserving, so no verdict may move. Gate on make test plus OOLD_SCHEMA_DIR=../oold-schema uv run pytest -m parity.

Best done after #118 and #119, which rewrite parts of context_resolution.py and resolve.py and would otherwise conflict with the renames here.