Harden pcb import passive value parsing

[akhilles]

Note

Low Risk
Localized parsing changes with expanded unit tests; risk is mainly small behavior shifts for ambiguous value strings during import.

Overview
Improves pcb import passive (R/C) value parsing so generic auto-promotion is more consistent across real-world BOM/value strings.

Parsing is hardened by tokenizing on additional separators (e.g. /, parentheses, brackets), normalizing Unicode and locale variants (Greek mu, ohm symbol variants, comma decimals), supporting resistor R10-style fractional notation, and attempting merged-token parses while avoiding accidental refdes/package merges (e.g. standalone R + 0402). Tests are expanded to cover these cases, and the fix is documented in the Unreleased changelog.

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