docs: propose the S3 versioning design (per-key version tree)

The implementable spec for S3 bucket versioning, built on the per-key
version-tree leaf proposed in the PR #2 architecture review (r3440362078),
which supersedes architecture.md §3's composite-key invertedVersionId
encoding: seq (ordering, internal) and version_id (identity, "null" or a
ULID token) are separate fields, so the null version's replace-in-place
semantics fall out structurally instead of via sentinels.

Covers the data model (ObjectLeaf / prev sub-MST keyed by inverted seq),
the token grammar, the write rule per versioning state, read/delete/list
semantics pinned against the versitygw conformance suite, reference-index
interplay, and a file-by-file implementation map.

Design doc only — the implementation lands in a follow-up PR once this is
ratified. (The design has been validated end-to-end by a prototype in this
worktree: unit suite plus the upstream versioning conformance categories
against the live smelt stack.)

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: frrist