internal/tags: route SerializeTags through TagSet

[th0114nd]

SerializeTags previously hand-unrolled its own compare-and-swap sort
for 1-4 tags, duplicating the tuned sorting network already
maintained in TagSet.Sort(), and its >4 tag branch always
heap-allocated the pairs TagSet before also heap-allocating the
output buffer (2 allocations for any tag count above 4).

Adds gatherValidTags/numValidTags, shared helpers that gather tags
into a caller-provided TagSet (typically backed by a small stack
array for tag counts up to 16, falling back to a single exact-size
heap allocation otherwise) and sort via the real TagSet.Sort().
SerializeTags now gathers this way and calls TagSet.Serialize,
reducing to exactly one allocation for any tag count that fits the
stack tier, instead of two, while removing the duplicated sorting
logic.

These helpers are also intended to back a hash-based lookup path in
a follow-up change, so both can share the same gather/sort machinery
from the start.

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

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