schema: add index_filter to get_interface_statistics (#272)

[AdamRickards]

Draft for sidecar live proof; do not merge.

Adds method-level index_filter: '^\d+/\d+$' to get_interface_statistics (schema YAML only), aiming for physical-only rows across protocols (#272).

Offline (tested): method index_filter is applied only in _resolve_intent when index is a list; _pipeline_egress / _shape_table_output do not read it. An OfflineHIOS fixture with cpu/* and vlan/* ifTable rows returns the same rows before and after this change, so offline shows the filter is not applied on reads.

Live (untested, pending): sidecar get_interface_statistics.read mops/snmp/ssh row counts and parity_diffs on this head vs main.

• pytest tests/: pass
• scripts/ci_offline.sh: pass
• generate_floors_board.py --check: pass

Related #272

[AdamRickards]

Sidecar live proof — get_interface_statistics.read trace:true, head 22ae25a vs main 9ee450d.

protocol	main	this PR
mops	ok, n=36 (incl. 1 cpu + 7 vlan rows)	ok, n=36 (same rows)
snmp	ok, n=36 (incl. 1 cpu + 7 vlan rows)	ok, n=36 (same rows)
ssh	ok, n=28 (physical only)	ok, n=28
parity_diffs	27 (utilization_interval mops vs snmp, capped)	27 (same)

Result: fail vs #272 floor (all protocols physical-only). Live matches offline: method index_filter is not applied on reads. No change from this PR. Needs an Engine read-side filter (see park row), not schema YAML.

Draft; do not merge.