feat(core): virtual file system and Swift string semantics

The core gains Swift's virtual file system (paths, case-insensitive
lookup that keeps the spelling, quotas, atomic changes, the defaults of
a new device) and the string behaviour the ported code relies on without
saying so. Both are measured in the Swift oracle: 11 file system cases
(the three Swift tests among them) step by step, and trimming, lower
case, equality, ordering and Base64 case by case — whitespace and control
characters over all of Unicode.

Found by measuring:
- Swift trims NEL and ZERO WIDTH SPACE, not the BOM; JS trim() does the
  opposite. The .fls import now trims with swiftTrim.
- lowercased() maps scalar by scalar: a word-final sigma stays σ.
- Foundation decodes "====" and "Y===" as empty data.
- Foundation on Linux counts 24,735 plane-14 scalars as control
  characters, a table artefact; the port keeps Unicode Cc+Cf.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

packages/core/test/strings.cases.json

@@ -0,0 +1,130 @@
+{
+ "trim": [
+  " a ",
+  "\t\na\r",
+  "\u0085a\u0085",
+  "\ufeffa",
+  "\u00a0a\u00a0",
+  "\u2028a\u2029",
+  "\u3000a",
+  "\u200ba",
+  "\u001ca",
+  "\u000ba\f",
+  " \u0301a",
+  "a \u0301",
+  "\u180ea",
+  "\u1680a"
+ ],
+ "lowercased": [
+  "ABC",
+  "\u03a3\u0391\u03a3",
+  "\u0391\u03a3 x",
+  "\u0130",
+  "\u1e9e",
+  "\u01c5",
+  "\u00c4\u00d6\u00dc",
+  "\u212a"
+ ],
+ "equal": [
+  [
+   "\u00e9",
+   "e\u0301"
+  ],
+  [
+   "\u00c5",
+   "A\u030a"
+  ],
+  [
+   "\ufb01",
+   "fi"
+  ],
+  [
+   "K",
+   "\u212a"
+  ],
+  [
+   "\u03a9",
+   "\u2126"
+  ],
+  [
+   "a",
+   "A"
+  ],
+  [
+   "\u00e9",
+   "\u00e9"
+  ]
+ ],
+ "sort": [
+  [
+   "b",
+   "a",
+   "B"
+  ],
+  [
+   "\uff5e",
+   "\ud83d\ude00"
+  ],
+  [
+   "e\u0301x",
+   "\u00e9a"
+  ],
+  [
+   "a",
+   "A",
+   "\u00e1",
+   "Z"
+  ],
+  [
+   "file10",
+   "file2",
+   "File1"
+  ]
+ ],
+ "base64": [
+  "aGVsbG8=",
+  "aGVsbG8",
+  "aGVs bG8=",
+  "aGVsbG8=\n",
+  "aGVs=bG8=",
+  "====",
+  "",
+  "aGVsbG8==",
+  "YQ==",
+  "YQ",
+  "Y",
+  "!!!!YQ==",
+  "YQ==YQ==",
+  "YWJj",
+  "Y\u00e9Q==",
+  "YQ=\n=",
+  "Y===",
+  "YQ=",
+  "YWI=",
+  "YW==",
+  "=YQ=",
+  "A===",
+  "AA==",
+  "AAA=",
+  "AB==",
+  "ABC=",
+  "YQ===",
+  "YQ==\u0000",
+  "Y=Q=",
+  "YWJjZA",
+  "YWJjZA=",
+  "=",
+  "==",
+  "==="
+ ],
+ "controlCharacters": [
+  "a",
+  "a\u200b",
+  "a\u007f",
+  "a\u00ad",
+  "a\u0085",
+  "\ufeff",
+  "a\ud83d\ude00",
+  "a\ue000"
+ ]
+}