Make explicit over-budget profile behavior match D6

Description

Release review finding P1.5 remains live at src/meraki_dashboard_exporter/collectors/manager.py:638-649. The startup gate rejects only an unset profile, while explicitly selecting standard or full can acknowledge the same over-budget solved plan without changing its demand. Reconcile validation and diagnostics with frozen decision D6: explicit profile selection is required above the computed threshold, priority 1 and 2 retain floors, and lower priorities shed when the selected profile still exceeds budget.

Acceptance Criteria

• #1 Unset adaptive over-budget plans fail with actionable diagnostics naming measured demand and profile options
• #2 Explicit availability, standard, and full profiles each follow the documented D6 floor and shedding behavior when over budget
• #3 Fixed scheduler mode does not apply adaptive demand validation
• #4 Tests cover unset and all three explicit profiles, and docs describe the selected-profile contract

Definition of Done

• #1 just check (ruff format –check, ruff check, mypy, generated-doc drift, offline API conformance, and the marker-filtered pytest run with the 80% coverage floor — this is exactly what the CI test job runs)
• #2 just gen, when metrics, config, endpoints, collectors, the settings schema or the chart config changed — just check includes the drift gate and CI fails the build on it
• #3 Grafana queries in grafana/dashboards/*.json and grafana/alerts/ updated, if a metric or label name changed

Implementation Plan

Root-owned manager seam: reconcile explicit adaptive profile selection with D6 by proving the selected profile applies its priority floor and shedding policy; retain fixed-mode behavior and update tests/docs.

Implementation Notes

D6 behavior was retained and pinned across unset, availability, standard, full, and fixed mode. Explicit profiles solve and apply their floor/shedding contract rather than being treated as a demand-reduction claim. Integrated gate: 2827 passed.

Final Summary

Aligned the explicit-profile contract with D6 through full profile/fixed-mode regression coverage and documentation; just check and just ci pass.