bug: preserve unusual keys, empty containers, big ints, emoji, and aliases across the save round-trip

[bosiakov]

• Encode flat keys with backslash escapes (\. \[ \) so map keys containing literal . (e.g. app.kubernetes.io/part-of) and empty-string keys ("": value) round-trip without splitting or rejection
• Skip empty-container placeholder entries in PatchNodeTree when the tree already holds a matching mapping or sequence, instead of overwriting with a literal '{}' / '[]' string scalar
• Set Value to empty for nil scalars in flattenValues so convertValue returns nil and yaml.v3 emits null / ~ / blank instead of the string <nil>
• Treat tree values null / ~ / "" as equivalent to an empty entry Value when checking valueUnchanged, so unedited null leaves keep their source style
• Skip SplitInlineComment when the editor text matches the loaded value verbatim, so a literal # inside a quoted string stops being misread as an inline comment
• Add a raw-equals-load-form fast path in collectOverrides so multi-line literal blocks whose own content starts with # round-trip without the leading line being hoisted into a YAML head comment
• Include scalar-leaf entries with empty Value in customMap so populateColumnOverrides no longer falls through to SerializeNodeSubtree, which was YAML-encoding empty strings as the four-character text "" and saving them as "\"\""
• Thread loadedValues through OverridesToYAML so the empty-value round-trip path is scoped to keys actually loaded from the file, preventing chart defaults like acl.userSecret: "" and master.persistence.medium: "" from leaking into the saved override
• Pull literal scalar text from NodeTree after flatten so chartutil's float64 cast on integers above 2^53 stops corrupting values like 9007199254740993 to 9.007199254740992e+15
• Run rewriteValuesFromNodeTree on the merge load path too, since the UI loads override columns through LoadAndMergeCustomValues
• Post-process the rendered YAML to convert yaml.v3's \Unnnnnnnn and \unnnn escape sequences inside double-quoted scalars back to literal Unicode, so emoji and supplementary-plane characters survive yaml.v3's forced double-quoted escape policy
• Make effectiveComments walk the tree read-only via findParentSlotReadOnly so comparing comment state on an aliased leaf no longer breaks the alias as a side effect
• Expose viaAlias from effectiveComments so PatchNodeTree treats alias-resolved comments as effectively unchanged, suppressing a same-position comment write that would otherwise expand the alias
• Preserve Value on custom-only empty-container entries in RebuildUnifiedEntries so they pass IsFocusable, survive collectOverrides, and stay in the saved file
• Drop the keyNode.Value == "" skip in parseComments and the position walker so comments on legitimate empty-string keys ("": value) attach to the right flat key
• Switch UI helpers to escape-aware FlatKey methods, fix anchor membership depth to use FlatKey.Depth, and use escape-aware splitting in suggestAnchorName
• Add tests covering FlatKey escape round-trip, empty/dotted/backslash key save, alias preservation, big-int precision, emoji escape removal, multi-line literal blocks, and custom-only empty-container preservation