refactor(once_map): build the table before locking it and simplify lookups

[orthur2]

Summary

FromIterator fills a local table and wraps it in the mutex once it is complete. insert(&mut self) was its only caller and locked a table nobody else could reach; a duplicate key now replaces its entry in place because no lock is held.

get_or_insert returns the entry and compute and try_compute check the cell themselves, replacing the Lookup enum and classify; the function now has the same shape as the singleflight group, including the note on where duplicate keys drop. A new test drops a duplicate key whose destructor discards another key, on both the pending and the ready path, so that order cannot regress silently. The entry is bound inside a block so that its scope ends before the await, which keeps the compute and try_compute futures at 192 and 176 bytes. get does its own lookup and remove_entry returns straight after the removal. The removed layers, the Entries alias, and a comment about a "write lock" were left from the sharded table, where the public map and the shard were different types. Every value is still cloned after the table lock is released, and public behavior is unchanged.

The unit tests use the imported pin! and Poll, pin futures where they are built, and check the release flag with a plain conditional; the lookup bench comment no longer describes a "ready index". Existing tests cover duplicate keys in collect(), cleanup of abandoned entries, and growth under concurrent compute. A new bench measures building a map from an iterator, which had no measurement before.

Benchmarks

main is 44fc2d8. Apple M5, macOS 26.4, Rust 1.96.0. cargo x bench --bench primitives -- --threads 1 with fixed sample counts, 100 samples of 64 iterations for the build rows and 200 samples of 256 for the lookup rows, both binaries alternating for ten rounds. Each cell is the median of the nine rounds after the first.

Benchmark	main	This branch	Change
once_map::build::collect_distinct_keys/64	957.4 ns	762.1 ns	-20.4%
once_map::build::collect_distinct_keys/1024	14.42 µs	11.40 µs	-20.9%
once_map::build::collect_duplicate_keys/64	1.22 µs	0.99 µs	-18.9%
once_map::build::collect_duplicate_keys/1024	18.87 µs	15.36 µs	-18.6%
once_map::lookup::get_hit_same_key	6.45 ns	6.45 ns	-0.1%
once_map::lookup::get_hit_distributed/1024	6.93 ns	6.94 ns	+0.1%
once_map::lookup::get_miss_distributed/1024	6.45 ns	6.46 ns	+0.2%
once_map::lookup::compute_hit_same_key	9.54 ns	9.54 ns	0.0%
once_map::lookup::compute_hit_distributed/1024	11.81 ns	11.66 ns	-1.3%

Building from an iterator saves one uncontended lock and unlock per element plus a second table probe for duplicate keys. The lookup rows are within one or two 0.16 ns timer steps of main.

Validation

• cargo x test (594 passed), cargo x check (26 feature configurations), and cargo x lint
• Miri on the once_map unit tests

Follow-up to #345.