chore: roll out ecosystem code-style policy (swift-format + SwiftLint + comment-ratio check)

[gsdali]

What

Roll out the ecosystem's proposed code-style policy to this repo, the second pilot after
OCCTSwiftScripts (#114 /
#115), replicating that pattern
exactly.

Proposal doc: ecosystem docs/code-style-policy-proposal-2026-08.md

This repo's facts

• ~699 Swift lines across ~6 files: Sources/SwiftJWW/JWW.swift, DXFWriter.swift,
  Reader.swift, Sources/jww2dxf/main.swift, Tests/SwiftJWWTests/SwiftJWWTests.swift. Very
  small: full sweep, trivially so. No exemption manifest needed.
• Zero first-party C++ bridge files, confirmed. No clang-format needed.
• Already has 6 mandatory OKF policies onboarded (okf/policies/). This adds a 7th.
• No existing .github/workflows/ CI today. This bootstraps CI from nothing.

Scope for this repo

• .swift-format (100 col, 4-space, copied verbatim from OCCTSwiftScripts) plus a full
  swift-format --in-place sweep of Sources/ and Tests/.
• .swiftlint.yml, scoped to only_rules: [orphaned_doc_comment] (same reasoning as
  OCCTSwiftScripts: SwiftLint's default rule set fights swift-format's formatting opinions and
  overlaps the ecosystem's own code-structure policy on complexity).
• Scripts/comment-ratio-check.sh: report-only nudge (never fails), bash to match this repo's
  own tooling convention.
• .github/workflows/code-style.yml: swift-format lint --strict and swiftlint lint --strict
  as blocking gates, comment-ratio as a non-blocking step.
• okf/policies/code-style.md, linked from okf/index.md, this repo's 7th mandatory policy.
• Hand-fix every remaining swift-format lint --strict / swiftlint lint --strict finding after
  the automatic sweep, the same way OCCTSwiftScripts#115 did.

Explicitly out of scope here

• clang-format: no first-party C++ in this repo.
• The Swift-symbol docs-drift checks proposed for OCCTSwift in the wider proposal (section 3b):
  not part of this pilot-pattern rollout, same as OCCTSwiftScripts#114's own scoping.

Process

Filed per the ecosystem policy's own gate: no repo implements without a filed issue first. PR to
follow, referencing this issue. Holding for review before merge, not to merge.