internal/tags: add HashTags/TagSet.Hash for zero-alloc tag hashing

[th0114nd]

Adds HashTags(name, tags) and TagSet.Hash(name), which compute a
128-bit hash of the same canonical serialized form SerializeTags/
TagSet.Serialize produce, without ever materializing that string.

The serialized name+tags bytes are built into a stack-resident buffer
sized to the smallest of three tiers (128/256/512 bytes) that fits,
falling back to a single exact-size heap allocation for larger tag
sets. Tag pairs are gathered into a small stack array and sorted with
slices.SortFunc before serializing, so this performs zero heap
allocations for the common case (up to 16 tags, serializing to 512
bytes or less).

This is intended to back a hash-keyed sync.Map lookup for counters/
gauges, so that repeat calls to NewCounterWithTags et al. (cache hits)
no longer allocate on every call the way building the serialized
string today does.

Co-Authored-By: Claude Sonnet 5 noreply@anthropic.com

---

_{Stack created with GitHub Stacks CLI • Give Feedback 💬}

[th0114nd]

Superseded by #185 (reordered: unify SerializeTags before introducing HashTags, so HashTags doesn't need an immediate rewrite).