docs: record rules conformance, import contract, and round-trip law

Add docs/rules-conformance.md documenting how the repo conforms to the
vendored canon, the documentation.md vs dependency-free conflict and its
scoped resolution, the domain-mismatched rules that do not apply, and the
conscious deviations. Add docs/package-import-contract-status.md. State
the round-trip law (Lambda1/Lambda2) in DESIGN.md.

docs/DESIGN.md

@@ -240,3 +240,19 @@ source.
 Yams and its bundled `CYaml` / libyaml-derived C parser are compatibility
 references for PureYAML. They should guide test cases and behavior questions.
 They are not source material to copy into this package.
+
+## Round-trip law
+
+PureYAML keeps the parser (`PureYAML.parse`) and the emitter (`PureYAML.dump`)
+as separate hand-written implementations, not one invertible grammar. The
+guarantee they jointly hold is a fixed point over the emitter's normal form, not
+a bijection over arbitrary author formatting (the surface-to-model relation is
+many-to-one: spacing, flow vs block, and comments do not survive into the model):
+
+- Lambda1 (model fidelity): for a `Model.Value` `x`, `parse(dump(x)) == x`.
+- Lambda2 (idempotence): `parse(dump(parse(s))) == parse(s)` for any input `s`.
+
+These are enforced as gates, not hoped for: corpus and generated-document
+round-trip tests assert reparsed equality, and the empty-collection degenerate
+values are pinned as passing regressions. `dump` is total: no force-unwraps, and
+every empty collection renders to `[]` / `{}` at every position.

docs/package-import-contract-status.md

@@ -0,0 +1,13 @@
+# Package import contract status
+
+One row per target. The contract: the library target imports nothing beyond the
+Swift standard library; the test target imports only `PureYAML`, `Testing`, and
+`Foundation`.
+
+| Target | Allowed imports | Actual | State |
+|---|---|---|---|
+| `PureYAML` (library, `Sources/`) | Swift standard library only (no `Foundation`, no external packages) | zero `import` statements | matches |
+| `PureYAMLTests` (`Tests/`) | `PureYAML`, `Testing`, `Foundation` | `@testable import PureYAML`, `import Testing`, `import Foundation` | matches |
+
+Enforced mechanically by `scripts/check-forbidden-patterns.sh` (no `import` in
+`Sources/`) and `Package.swift` (`dependencies: []`).

docs/rules-conformance.md

@@ -0,0 +1,74 @@
+# Rules conformance
+
+This records how PureYAML conforms to the vendored Swift rules canon
+(`docs/rules-swift/`, mirror of `rules-swift`), including the few places where a
+canon rule does not apply to a dependency-free YAML library or conflicts with a
+project non-negotiable. Per `docs/rules-swift/core/rules.md`, conflicts are
+resolved by scope: the condition under which each rule applies is named, so
+exactly one governs any case.
+
+## Conformant (applicable rules)
+
+Every applicable rule passes its acceptance check. Mechanically verified:
+`check-style`, `check-namespacing` (now namespace-aware, catches one-type-per-file
+inside `extension` blocks), `check-forbidden-patterns`, `check-changelog-touched`,
+`check-roadmap`, `swiftformat --lint`, `swiftlint --strict`, `swift build`,
+`swift test` (277 tests). Concurrency, dependency-injection, cross-platform,
+framework-policy, namespacing (anchor choice), folder-grouping, domain-first, and
+the no-shortcuts mechanical tells are all clean.
+
+## Conflict: documentation.md Rule 1 vs the dependency-free non-negotiable
+
+`documentation.md` Rule 1 mandates a DocC catalog plus `swift-docc-plugin` as a
+**package dependency**. PureYAML's binding non-negotiable (`CLAUDE.md`,
+`AGENTS.md`) is **no external SwiftPM dependencies** (`dependencies: []`). Both
+cannot hold. Resolution by scope: the dependency-free invariant is the
+higher-stability project contract and wins. PureYAML is therefore exempt from
+`documentation.md` Rule 1's plugin requirement. API documentation lives in
+source `///` comments and `docs/`. If a maintainer later accepts a build-time
+plugin dependency, this exemption is revisited.
+
+## Not applicable: domain-mismatched canon files
+
+Several canon files describe other domains (OpenAPIKit data parsing, an
+After-Effects-style tool, app startup, UI). Their domain-specific clauses do not
+bind a dependency-free YAML library; the transferable clauses are satisfied.
+
+- `parsing-rules.md`: the CSV/XML/Excel/`Codable`-record and multi-encoding
+  `Data` clauses are N/A; the applicable parse-then-validate separation and
+  enum-with-associated-values clauses pass.
+- `validation-rules.md`: the OpenAPI component-store / `$ref` / type-erasure and
+  "validate-or-exclude every public type" coverage-gate clauses are N/A (the
+  input model is the 5-case `Model.Value` enum, handled exhaustively, so no input
+  is silently dropped). The applicable rule/validator/issue-path clauses pass.
+- `business-rules-and-constraints.md`: entirely N/A (no external services, auth,
+  PII, or compliance surface in an offline library).
+- `ui/*`, `openapi-generated.md`, `exp/app-target.md`: N/A (no UI, not generated,
+  no app target).
+
+## Conscious deviations (intent met another way)
+
+- `core/round-trip-transformation.md` "by construction" (one `ParserPrinter`):
+  PureYAML keeps a hand-written parser and emitter rather than a single
+  invertible algebra, because a bijective ParserPrinter is not a faithful model
+  of full YAML (block/flow, anchors, tags, chomping, comments; surface-to-model
+  is many-to-one). The law it protects is enforced by a fixed-point gate:
+  `parse(dump(parse(x))) == parse(x)` over the corpus and generated documents,
+  plus the empty-collection adversarial regression. The round-trip law is stated
+  in `docs/DESIGN.md`.
+- `code-style.md` file-naming: the repo uses folder-based bare names in some
+  folders (`Parsing/`) and dotted `<Namespace>.<Type>.swift` in others
+  (`Tagged/`, `Validation/`). Both are blessed patterns; the standing convention
+  is dotted for new namespaced files. A full rename to one pattern is deferred as
+  disproportionate churn against the highest-stability rename surface.
+- `documentation.md` Rule 14 (`///` on every public decl): the non-trivial public
+  entry points are documented; the remaining gap is the MAY-skip categories
+  (`Codable` conformance members, `CustomStringConvertible.description`,
+  self-describing data fields). Tracked, not blanket-filled.
+
+## Historical commits
+
+A few commits that predate the commit-message hook have non-Conventional or
+over-72-character subjects. They are already on `origin/main`; rewriting pushed
+history is out of scope. The `commit-msg` hook now enforces Conventional Commits
+and the 72-character subject limit going forward.