Distinguish Windows drive-relative, rooted and UNC paths in path_is_abs

[LunaStev]

path_is_abs returns true for any nonempty character followed by a colon, and recognizes a leading slash but not a leading backslash. This classifies C:relative and 1:relative as absolute while rejecting a full UNC path such as \\server\share\file. Windows drive-relative names must not be confused with a drive root. The function also needs an explicit target policy instead of applying drive syntax indiscriminately to Unix paths.

Source evidence at the head of #520:

• std/path/core.wave:41 — pub fun path_is_abs

Acceptance:

• Make the target-specific interpretation explicit in the implementation and short API comments.
• Cover relative names, drive-relative C:foo, drive-rooted C:\foo, UNC paths, root-relative paths and ordinary Unix absolute paths.
• Require a valid drive prefix and distinguish rooted paths from fully qualified paths where needed.
• Keep byte-boundary reads safe for empty and one-byte strings.

API/reference context: Windows fully qualified and relative paths.

Audit status: identified by static source inspection; the scenarios above have not been executed during this audit. The permalink fixes the reviewed revision; this report does not claim the defect was introduced by #520.