docs(swarm): add §9 Implementation status to SWARM_SPEC.md

[Dewinator]

Summary

• Adds §9 Implementation status — a per-phase table mapping each Swarm Foundation Plan phase to its tracking issue and merged commit on `main`. Lets a reader (or future implementer in another language) tell at a glance which sections of the spec are wired vs. still paper.
• Updates the now-stale header `Status:` line ("spec-only") to reflect that phases 0–3 are all merged.
• Listed phases 4–9 as `not yet issued` rather than inventing speculative issue numbers — keeps the autonomy loop from creating duplicates if Reed later opens those issues by hand.

Why now

The autonomy loop just shipped phase 3d (mig 071, PR #92) and the spec header still claims `spec-only`. New readers landing on the spec from a search result would conclude — incorrectly — that nothing is built.

This is a strict docs-only delta inside an existing file: zero code, zero behavior change. Safe to merge in any order with the open code PRs (#96, #97, #98).

Test plan

• `git diff --stat` — only `docs/SWARM_SPEC.md` touched (+35 / −2)
• Markdown anchor for the new `§9 Implementation status` link in the header verified by section heading slug rules (lowercase, hyphenated)
• Visual check on GitHub — table renders, anchor jump works

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