feat: expose dense map capacity footprint

[hurricane1026]

Summary

• expose policy-aware entry/value capacities and direct allocation footprint
• project the exact direct footprint of reserve(count) plus reserve_values(count) without allocating
• share checked normalization/load-factor rules with real growth and reject value-index overflow
• preserve value-only reservations made before bucket allocation

This is the container prerequisite for ClapDB timeseries FrequencyDict state reservation and reconciliation. Nested key/value-owned payloads are intentionally excluded.

Validation

• Containa full test binary: 313 test cases, 1,595,365 assertions passed
• ClapDB clean alltests consumer build: in progress locally