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>
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