fix(capabilities): gate ibis-sqlite MA multiplier durations — clean BackendCapabilityError (item 99)

[discreteds]

Summary

Closes backlog item 99: the last raw-error leak in the datetime rounding surface.

dt.truncate("2d") / round_dt("3h") / ceil_dt("12mo") / floor_dt("2w") on ibis-sqlite reached ibis's TimestampBucket (multiple > 1), which has no sqlite compilation rule — raising a raw native OperationNotDefinedError instead of the clean BackendCapabilityError the capability spine promises. Every other backend/dialect honored the multiplier or raised the declared error; ibis-sqlite was the sole silent leak.

Root cause

• The MA api-builder stores only the raw duration string in the node's unit option ("2d"); the multiplier is derived later by the backend body via parse_ma_unit. There is no multiple option on the MA node.
• The visitor's option gate resolves capability_for(MA_FKEY, "unit", ..., option_value="2d"). The existing ibis-sqlite facts in capabilities/datetime/options.py are exact-value facts for the single-unit domain ("1h", "1q", …) — "2d" matched nothing, so the call passed through to x.bucket().
• The direct Substrait ops (round_temporal/round_calendar) already gate multiple on ibis-sqlite (rounding.py); only the MA-wrapper path was ungated.

Fix

• 4 new DURATION_MULTIPLIER value-class facts (param="unit", ibis, ibis-sqlite) for TRUNCATE/ROUND/CEIL/FLOOR in capabilities/datetime/value_classes_ma.py. The visitor option gate resolves value-class facts, so the clean BackendCapabilityError now fires at build time. "1d"-style single units do not match the class predicate (multiplier ≥ 2) — unaffected.
• Integrity guard: the matrix-exercise class arm in test_option_fact_integrity.py now exempts class facts whose (backend, dialect) the 4-fixture matrix cannot instantiate (mirrors the existing _MATRIX_UNREACHABLE_DIALECT_FACTS exact-arm precedent), with a new staleness guard.
• Dedicated gate tests: TestMaMultiplierIbisSqliteGate builds a real ibis-sqlite backend — 4 multiplier durations raise BackendCapabilityError, 4 single-unit durations still compile/evaluate (no over-gating). RED→GREEN: the tests failed with the raw OperationNotDefinedError before the facts landed.
• Registry cleanup: the 2026-08-16 KNOWN_GAPS entry (added when the gap was only disclosed) is retired — the fact supersedes it. Coverage docs regenerated.

Verification

• Full suite: 23119 passed, 81 skipped, 3470 xfailed (baseline +13 = the new gate tests).
• Targeted (877 passed / 72 xfailed): test_datetime_rounding.py, test_option_fact_integrity.py, test_datetime_value_class_dispatch.py, test_arg_types_datetime.py, test_datetime_unit_dispatch.py, test_expression_coverage_doc.py, test_capability_value_class_gate.py.
• Coverage artifacts byte-equal after regeneration (test_expression_coverage_doc.py).
• Ruff: clean on all changed files.
• Empirically confirmed pre/post: truncate('2d') raised OperationNotDefinedError: Compilation rule for 'TimestampBucket' operation is not defined → now BackendCapabilityError; truncate('1d') unchanged.

Spec: mountainash-central/04.planning/mountainash/superpowers/specs/2026-08-18-ibis-sqlite-timestampbucket-raw-error-leak-design.md
Plan: mountainash-central/04.planning/mountainash/superpowers/plans/2026-08-18-ibis-sqlite-timestampbucket-raw-error-leak.md