refactor phase 6: extras boundaries + the layer contract

[hussainsultan]

Phase 6 — the final planned phase of the refactor, stacked on #302. The structural rules the whole stack worked toward are now machine-enforced, and the optional extras stand on the public surface. Full suite: 1745 passed, exit 0; make examples-core green.

Extras stand on the public surface

• server no longer imports agents: the chart endpoints resolve the agents chart generator at call time and answer HTTP 501 with an install hint when the agent extra is absent — pip install 'boring-semantic-layer[server]' gives you a server that works, not an ImportError.
• query.find_time_dimension is public — the server and the MCP backend were importing the underscore name.
• chart/utils uses the op metadata protocol (source.get_dimensions()) instead of reaching into ops privates. That forced the protocol to be complete: SemanticGroupByOp/SemanticProjectOp join the pass-through mixin — a real regression (time-series charts rendering as bars because GroupBy lacked get_dimensions) was caught by the chart suites mid-change and drove the fix.

The layer contract, enforced

The SCC ratchet graduates into the full structural contract in test_import_graph.py:

• test_layer_contract — every import edge (lazy included) must point to an equal or lower layer: primitives → analysis → compilers → ops → expr/format → api/query/yaml → serialization → extras + root facade. A new top-level module must declare its layer; the test refuses to guess.
• test_extras_do_not_import_core_privates — chart/agents/server may import only public core names and modules (the _xorq flavor shim is the one documented exception).

Both pass on the current graph with zero allowlisted violations — the contract describes what the stack built, not an aspiration.

Dependency posture

ibis-framework capped <12 (core subclasses Relation and walks Deferred internals; a new major gets matrix-tested by the weekly IBIS_VERSION=all job before the cap moves). Lockfiles regenerated.

Housekeeping

A broad git add briefly swept local working files (CLAUDE.md, a CSV, an embedded repo, three WIP example scripts — one of which imports the malloy package that only exists on feat/malloy-interop and would have broken make examples in CI). All un-committed in a follow-up; git-catalogs/ and flights.csv are now gitignored so it can't recur.

Where the plan stands

This completes the six planned phases (#295→#303): CI safety net → dead code + SCC rim → primitives + typed errors → DAG → ops split → preagg decomposition → serialization consolidation → API surface → boundaries + contract. Remaining named follow-ups, none blocking: phase-4-interior decomposition of the per-source preagg loop, ~40 broad handlers in the preagg path, fluent time_grain/having methods, the index() wrapper, the drifted twin test files, and the expr.py docstring pass.

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