The decoder's `parse_lola_binary` inserts the same "VAR:LEVEL"
string into ~92k cell maps for every one of ~60 GRIB2 messages in
a chain step — previously a String::clone per cell, yielding
~5.5M heap allocations and frees per hourly run. Switching
`CellValues` from `HashMap<String, f32>` to
`HashMap<Arc<str>, f32>` makes each insertion a single atomic
refcount increment: the string is allocated once per message, and
`Arc::clone` is cheap from there.
Callers were mostly unchanged — `cell.get("...")` still works via
`Arc<str>: Borrow<str>`. Two touch-ups needed: native_duct.rs had
a couple of `cell.get(&format!(...))` lookups that were passing
`&String` (triggering the wrong Borrow impl) — switched to
.as_str(), and the test fixtures' `.insert(format!(...), _)` now
need a `.into()` to coerce into `Arc<str>`. Zero behavior change.
Graham McIntire
83aa3115df