Backfill standalone documents for protocol 0.1 and 0.2

[koishore]

What

Adds versions/ with a standalone document of record per prior protocol version, alongside the frozen v0.3 spec.md:

• versions/spec-v0.1.md — protocol 0.1 (initial; approvals fingerprint-bound only)
• versions/spec-v0.2.md — protocol 0.2 (what the reference delego 0.2.x implements today)
• versions/README.md — index + derivation/maintenance rules

Also links them from the README's Status & versioning section (and corrects its stale "v0.3 — draft" status to "frozen"), and records the change under [Unreleased] in the changelog as editorial.

Why

spec.md is frozen at 0.3 and tags clauses inline ((since 0.2), (0.3, draft …)). An implementation declaring 0.1 or 0.2 (delego.__protocol_version__ is "0.2") had no readable spec of record without filtering those tags by hand.

How the snapshots were produced

Derived from the frozen v0.3 document by removing every clause tagged above the target version — not resurrected from git history: no standalone 0.2 doc ever existed, and the original 0.1 draft carries the since-corrected §8.1 truncation claim and pre-genericization examples. Each snapshot's banner states it was backfilled retroactively and points to git history for the originals.

Notable choices:

• Section numbering aligned across all three documents; sections that exist only in later versions (§5.1, §7.1, §8.3, §9) are stubbed with pointers to spec.md.
• v0.1 documents its real gaps (no intent guard, no single-use; P2/P3 not upheld) in warning boxes rather than omitting them.
• §9 stubbed in both — the pre-0.3 token sketch is not republished under any version banner.
• Backfilled v0.2 content cross-checked against the reference implementation's behaviour (engine.py resolve guards, audit.py verify).

No protocol change

spec.md is untouched; normative content remains defined exclusively there. CI is unaffected (validate.py checks JSON vectors only).