chore(#9): roll out ecosystem code-style policy

[gsdali]

What & why

The second pilot repo for the ecosystem's proposed code-style policy
(ecosystem docs/code-style-policy-proposal-2026-08.md),
following the reference implementation already trialled in
OCCTSwiftScripts#114 /
#115. This repo is tiny
(~699 Swift lines across ~6 files), so it gets the same treatment: a full sweep into
compliance in one PR with an immediate zero-tolerance blocking gate, not a gradual
exemption-manifest rollout.

Adds:

• .swift-format (100 col, 4-space, the ecosystem's one deliberate divergence from
  Google's own 2-space default), copied verbatim from OCCTSwiftScripts, plus a full
  swift-format --in-place sweep of Sources/ + Tests/.
• .swiftlint.yml, deliberately narrow (only_rules: [orphaned_doc_comment]). SwiftLint's
  default rule set duplicates swift-format's formatting opinions (can disagree with them on
  the same line) and separately opens a large code-quality/complexity surface
  (identifier_name, cyclomatic_complexity, function_body_length, nesting, ...) that
  overlaps the ecosystem's own code-structure policy rather than this one. Scoped down to
  the one rule that catches something swift-format has no equivalent for.
• Scripts/comment-ratio-check.sh: report-only nudge (never fails), matching this repo's
  own bash-script convention.
• .github/workflows/code-style.yml: swift-format lint --strict and
  swiftlint lint --strict as blocking gates, comment-ratio as a non-blocking step. Pure
  text analysis, no swift build needed. This repo had no .github/workflows/ before this
  change, so this bootstraps CI from nothing.
• okf/policies/code-style.md: this repo's 7th mandatory policy, linked from
  okf/index.md.
• 10 by-hand fixes for lint-only findings swift-format --in-place can't auto-fix:
  8x BeginDocumentationCommentWithOneLineSummary (blank /// line inserted after the
  one-sentence summary, in JWW.swift x3, DXFWriter.swift x3, Reader.swift x2), and
  2x EndOfLineComment (moved an over-length trailing comment to its own line, in
  DXFWriter.swift and the test file). swiftlint's orphaned_doc_comment found zero
  violations; no doc comments needed moving or deleting.
• Cleared pre-existing em-dashes from every file touched in this PR, per the ecosystem's own
  writing-style policy ("clear them from any file you are already editing"). No file was
  opened solely for this; it rode along with files already being formatted for the sweep.

Explicitly out of scope here (see #9): clang-format (zero first-party C++ in this
repo, confirmed), and the Swift-symbol docs-drift checks proposed for OCCTSwift in the
wider proposal, same scoping OCCTSwiftScripts#114 used.

Zero behavior change. Every edit is formatting or doc-comment structure only; no
renames, no signature changes. Verified directly, not just asserted:

• swift-format lint --strict and swiftlint lint --strict both pass locally with zero
  violations.
• Scripts/comment-ratio-check.sh passes (no file at or above the 1.0x threshold).
• swift build and swift test both run before AND after the sweep: identical result
  both times, build succeeds, all 5 tests in SwiftJWWTests pass.
• No Package.resolved exists in this repo (no dependencies), so nothing to check there.

Closes #9

Checklist

• New or changed behavior is covered by a unit test in the same PR (not just manual
  verification); see SecondMouseAU/OCCTReconstruct#397
  for the ecosystem-wide test-coverage standard this is piloting.
  N/A here: no behavior changed (formatting/docs/CI only), verified via the existing
  SwiftJWWTests suite rather than new tests, since there is no new behavior for a new
  test to cover.

Notes for the reviewer

Per the ecosystem's own gate (no repo implements without a filed issue first), this was
filed as #9 before any of this landed. Holding for review before merge, not to merge.