fix: walk join branches in tag_handler.extract_metadata

[hussainsultan]

Summary

• tag_handler.extract_metadata walked only the source chain looking for the innermost SemanticTableOp. For joined models the tag tree branches via SemanticJoinOp.left / .right (no source), so the walk stopped at the join op and the catalog summary returned "0 dims, 0 measures" and empty dimensions/measures arrays — even though every leaf SemanticTableOp's metadata was intact further down the tree.
• Rewrote the walker to recurse through source, fan out into both branches of any SemanticJoinOp, and union dim/measure/calc-measure names from every SemanticTableOp leaf. Inside a join subtree, names are prefixed with the leaf's table name so the catalog summary matches the field names a joined SemanticTable exposes (e.g. flights.flight_count); single-table models still emit flat names so the existing base-model tests continue to hold.
• Calc measures are now also surfaced in the summary when present.

Scope

Audited the rest of the codebase for the same source-only blind spot — none found:

• from_tag_node walks source only, but that's intentional: it strips off query wrappers down to the base op, then the _reconstruct_join reconstructor handles left/right itself.
• reemit operates on tag_node.parent, doesn't traverse metadata.
• chart/utils.py walkers operate on live op trees and use _get_merged_fields / _find_all_root_models, which already handle joins.
• All linear ops (SemanticFilterOp, SemanticProjectOp, SemanticGroupByOp, SemanticAggregateOp, SemanticMutateOp, SemanticOrderByOp, SemanticLimitOp, SemanticUnnestOp, SemanticIndexOp) have a single source and pass through correctly.

Test plan

• nix develop .#impure + uv run python -m pytest src/boring_semantic_layer/tests/test_xorq_tag_handler.py — 9 passed (8 existing + 1 new)
• nix develop .#impure + uv run python -m pytest src/boring_semantic_layer/tests/test_xorq_tag_handler.py src/boring_semantic_layer/tests/test_xorq_rebuild.py src/boring_semantic_layer/tests/test_xorq_convert.py src/boring_semantic_layer/tests/test_xorq_integration.py — 31 passed, 14 skipped, 1 xfailed
• New regression test test_extract_metadata_walks_join_branches covers a two-arm join_one chain wrapped in a query and asserts the union of prefixed dim/measure names ({t1.id, t1.name, t2.id, t3.id, t3.extra} and {t1.count, t2.total, t3.extra_count}).

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