Extract shared YAML-scan helper for scanner and editor

[VPDPersonal]

Summary

• Extract the duplicated YAML-scan primitives shared by the graph scanner and the repair flow into one new internal static SerializeReferenceYaml helper: the --- !u!N &id document-header regex, the RefIds: block lookup, the inline class/ns/asm type grammar plus its TryParseInlineType, IndentOf and FindEntryEnd.
• SerializeReferenceGraphScanner and SerializeReferenceYamlEditor now both call the shared primitives, so the graph window and the missing-reference repair flow can no longer silently disagree about Unity's RefIds shape (quoting, indentation, sentinel handling).

Notes for review

• The shared IndentOf counts tabs as well as spaces (matching the editor's existing implementation and both readers' \s* indent regexes). The scanner's old copy counted spaces only; since Unity always emits space-indented YAML this is behavior-neutral on real assets and removes a latent inconsistency with the scanner's own regexes.
• The shared DocumentHeader regex uses named groups (class/id); the editor's FindMissingReferences / FindDocumentRange were updated from Groups[1] to Groups["id"] to keep reading the file-id anchor (not the class id).
• FindEntryEnd is unified on the headerIndex signature; the scanner's single call site was adjusted from passing i + 1 to i accordingly (same scanned range).
• The editor keeps thin private forwarders (IndentOf/FindEntryEnd/FindRefIdsStart/TryParseInlineType) delegating to the shared helper, so its many internal call sites stay untouched while the implementation is single-sourced.

Linked issues

Refs #49 - addresses review finding #49 (comment)