usage module: cosmetic follow-ups from review of #43

[chmmou]

Bundled Nice-to-have findings from the post-merge review of #43 (usage read module). Each item is non-blocking; ship at your convenience.

• internal/usage/usage.go:24-33 — Space struct keeps both UsedWebspace (the sum) and the four sub-buckets without a doc note that the sum equals the buckets. Add a one-liner so future readers don't double-count.
• internal/usage/usage.go:55-65 — Traffic.Day == 0 is overloaded (summary row vs. literal day-zero, the latter never occurs). Add a (t Traffic) IsSummary() bool helper to make caller intent explicit.
• internal/cli/usage.go:80-84 — --year / --month accept any int; out-of-range values are only rejected by KAS after a roundtrip. Pre-validate month ∈ 1..12 and a plausible year range for faster feedback.

[chmmou]

Status update (2026-05-09, against main at 28393d2):

All three findings still apply unchanged. Line numbers shifted slightly since the issue was filed:

• Item 1 — Space struct sum doc: internal/usage/usage.go:24-33 (unchanged). The struct docstring covers parsing/overflow but still does not state that UsedWebspace equals the four sub-buckets summed.
• Item 2 — Traffic.IsSummary() helper: internal/usage/usage.go:53-69 (was 55-65; shifted by helper-consolidation refactor refactor: consolidate getString/getFloat/getInt into soap.Value methods #65). The struct docstring already explains the Day == 0, AccountLogin set, Comment populated pattern; the helper itself is still missing.
• Item 3 — --year / --month pre-validation: internal/cli/usage.go:94-95 (was 80-84). Both flags are declared as plain IntVar and passed straight to usage.Client.Traffic without bounds-checking.

No body update needed; the issue scope is still correct. Keeping it open as Nice-to-have.