Break SDL↔processor cycle; add ADR-015 layering + size-cap gates

[Brad-Edwards]

Summary

• Breaks the aces_sdl ↔ aces_processor circular import. aces_sdl/validator.py imported aces_processor.semantics.{objectives,workflow} while aces_processor.{compiler,manager,planner,models} imported aces_sdl.*. The two SDL-language semantics modules (objectives.py, workflow.py) move from aces_processor/semantics/ to aces_sdl/semantics/; planner.py stays in aces_processor (it reconciles compiled resources, a processor artifact). The aces.core.semantics.{objectives,workflow} compatibility wrappers retarget to the new locations, so the public aces.* surface is unchanged.
• Adds ADR-015 and the policy gates that enforce it (tools/policy/repo_policy.py, config in tools/policy/adr_policy.yaml): a layering rule (aces_sdl/ may not import aces_processor[...], AST-scanned) and a 600-line cap on non-test source files under implementations/python/packages/, with a draining allowlist (tools/policy/oversized_allowlist.yaml) that must stay a subset of a code-anchored initial set and that can only be drained by actually splitting a file (not by editing the allowlist away). Malformed policy config (including an unparseable / non-mapping adr_policy.yaml) and out-of-tree changed/allowlist paths surface as structured failures (policy-config-malformed, policy-path-unsafe) rather than tracebacks; the config-wide reconciliation runs even on deletion-only diffs.
• Adds the MOD-001 ("Codebase Modularity And Layering") requirement (a new modularity-initiative phase in tools/policy/requirement_order.yaml) as the Ground Control anchor for issue improve-modularity #3's modularity work — this PR is the cycle-break + size-cap-gate part; the 14 file splits are improve-modularity #3's child issues, which drain oversized_allowlist.yaml one entry at a time (the tracker closes when it's empty). Also: CHANGELOG 0.14.0, ADR-015 added to the index + toctree, and a docs/api/{sdl,processor}-semantics.rst split.

Review context

The gc_codex_review loop ran 4 verification passes past its cap (cycles 17–20, 5/3/3/3 findings) — every finding is dispositioned with a code fix + test or a documented design decision; all four dispositions are recorded as comments on #38. Notable: cycle-19's "move validate_workflow_step_result to the processor for layering purity" fix broke a compat surface, so it was reverted in cycle-20's pass (the function is pure and doesn't violate the layering rule wherever it lives); cycle-20's "add owning-package re-export shims" was declined per ADR-009/010 (the aces.* namespace is the one stable shim layer — see ADR-015 §1). nox -s verify is green (hygiene · policy · lint · contracts · 809 tests + 50 new policy-tool unit tests · docs).

Test plan

• nox -s verify passes locally
• 50 unit tests in implementations/python/tests/test_repo_policy_tools.py cover: the layering rule (all four import shapes + prefix-boundary + out-of-scope); the size cap (over/under/allowlisted/test-excluded/non-Python); the drain rule (subset of the code constant; config cannot grow the locked set; premature drain rejected; legitimate drain passes); the stale-allowlist-entry rule (below cap / missing / replaced by an out-of-tree symlink); required ADR-015 config blocks; malformed config including a parse-broken / non-mapping adr_policy.yaml; the unsafe-path chokepoint; and config-wide checks running on an empty changed list (deletion-only PRs)
• aces.core.semantics.* namespace imports still resolve (no test changes needed); full pytest sweep (809 passed) exercises the moved modules via validator.py, compiler.py, manager.py
• CI green on this PR
• SonarCloud quality gate

Closes #38. Refs #3.