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>

tools/swift-oracle/run.sh

@@ -12,4 +12,6 @@ docker run --rm -v "$root:/work" -w /work/tools/swift-oracle swift:6.1-jammy sh
     swift build -q
     .build/debug/Oracle /work/packages/core/test/scenarios
     .build/debug/Oracle --corpus /work/corpus /work/corpus/corpus.swift.json
+    .build/debug/Oracle --vfs /work/packages/core/test/vfs.cases.json /work/packages/core/test/vfs.swift.json
+    .build/debug/Oracle --strings /work/packages/core/test/strings.cases.json /work/packages/core/test/strings.swift.json
     .build/debug/Oracle --path-folding /work/packages/fls/test/path-folding.cases.json /work/packages/fls/test/path-folding.swift.json'