`container_view::build_all` renders every container on every scan, and
each agent's row resolved its limits through `effective()`, which reads
and parses `resource-limits.json` from disk. That is one file read per
agent per scan of a file that is identical for all of them.
Split the resolution in two: `effective_from` takes an already-loaded
map, and `effective` keeps the read-then-resolve shape for the
single-agent callers (`write_dropins`, which runs once per spawn and
rebuild and has no map to hand). `build_all` now loads the map once at
the top — the same treatment `topology::read()` already gets there —
and calls `effective_from` per agent.
No behaviour change: the fallback matrix lives in `resolve`, which both
paths still go through, and its tests are untouched.
The now-redundant `limits_for` is gone; `effective_from` covers its one
caller.
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