fix: do not mutate the caller's exclude map in writeSubset, drop the global mutex

[burruplambert]

Summary

writeSubset writes the Header-Order:/PHeader-Order: magic keys into the caller's exclude map:

if exclude == nil {
    exclude = make(map[string]bool)
}
mutex.Lock()
exclude[HeaderOrderKey] = true
exclude[PHeaderOrderKey] = true
mutex.Unlock()

Callers pass shared package-level maps - respExcludeHeader from Response.Write, reqWriteExcludeHeaderDump from httputil - so that write both races with concurrent header writes and permanently leaks the two exclusions into every later write that uses the same map. An orderless write's output ends up depending on whether an ordered write happened earlier in the process.

The package-global RWMutex exists only to guard that mutation. It was added in 7abeb12 ("in order to avoid any conflicts in the exclude map when several tasks are run") and extended to SortedKeyValuesBy in 107c8e4. The cost is two global lock operations per header key on every sort, shared across all connections.

Changes

• writeSubset adds the magic keys to a small copy of the exclude map and never writes to the caller's map
• the mutex then guards nothing, so it and the per-key RLock/RUnlock in SortedKeyValues and SortedKeyValuesBy are removed
• tests: the caller's map is not mutated, and concurrent writes sharing one exclude map are clean under -race
• adds BenchmarkHeaderWriteSubsetParallel, since the mutex cost only appears under concurrency

Behavior

Wire output is unchanged for every fresh-process input. The only behavioral difference is that an orderless write no longer depends on the process's write history, which was unintended state leakage rather than a feature.

Measurements

Single-threaded BenchmarkHeaderWriteSubset is about -6%; uncontended atomics are cheap. Under BenchmarkHeaderWriteSubsetParallel on 2 cores it is -28% ns/op, and the gap widens with core count.

Note

TestHeaderWrite case 9 fails on master, before and after this change: the order map is built from the Header-Order: values verbatim while the comparator looks them up lowercased, so mixed-case order entries never match. Unrelated to this change and left alone, since fixing it changes wire behavior.