chore(rules): align lint config and pointer with canonical rules-swift

Make public/rules-swift the canonical source (per project direction) and adopt
its opt-in lint rules that PureXML lacked: empty_count, explicit_init, first_where,
sorted_first_last, unused_import, plus its todo disable. All pass with zero
violations on the existing sources.

PureXML keeps its coherent, stricter tuning where the canon is a self-described
'starting point': force_unwrapping/force_try/force_cast = error (safety), and
trailing commas mandated in both swiftformat (--commas always) and swiftlint
(mandatory_comma) to resolve the canon's formatter-adds vs linter-flags mismatch.
docs/rules/README.md now points at public/rules-swift and documents the tuning.

.swiftlint.yml

@@ -5,10 +5,18 @@
 # build plus test ships with the engine package.
 opt_in_rules:
   - force_unwrapping
+  # Adopted from the canonical rules-swift opt-in set.
+  - empty_count
+  - explicit_init
+  - first_where
+  - sorted_first_last
+  - unused_import
 disabled_rules:
   # Suggests Foundation's String(bytes:encoding:); PureXML is Foundation-free, so
   # String(decoding:as:) from the standard library is the correct conversion.
   - optional_data_string_conversion
+  # rules-swift disables todo; PureXML tracks TODOs in issues, not the linter.
+  - todo
 excluded:
   - .build
   - DerivedData

docs/rules/README.md

@@ -25,6 +25,11 @@ Project-specific overrides:
 The broader rule files are retained for detailed guidance. When they include
 generic examples, apply the PureXML-specific overrides above.
 
-Canonical Swift rules live at `/Volumes/Code/DeveloperExt/private/rules-swift`
+Canonical Swift rules live at `/Volumes/Code/DeveloperExt/public/rules-swift`
 (especially `validation-rules.md` and `parsing-rules.md`). Prefer that source
-when `docs/rules/` copies drift.
+when `docs/rules/` copies drift. PureXML's committed `.swiftlint.yml` and
+`.swiftformat` are the coherent project tuning of that canon: they adopt its
+opt-in rule set and shared formatter directives, keep stricter safety
+(`force_unwrapping`/`force_try`/`force_cast` = error), and resolve the canon's
+`--commas always` vs default-`trailing_comma` mismatch by mandating trailing
+commas in both tools.