fix: filter all-zero context_window frames to prevent Context Bar 0k flash

[KorenKrita]

Problem

Claude Code occasionally emits transient context_window status JSON frames where all usage fields are zero mid-session:

{
  "context_window_size": 200000,
  "total_input_tokens": 0,
  "total_output_tokens": 0,
  "current_usage": { "input_tokens": 0, "output_tokens": 0, "cache_creation_input_tokens": 0, "cache_read_input_tokens": 0 },
  "used_percentage": 0,
  "remaining_percentage": 100
}

These all-zero frames cause the Context Bar widget to briefly flash 0k/200k (0%) before the next valid frame restores the correct display. With refreshInterval: 10, the statusline polls frequently enough to sample these intermediate frames.

Observed in practice: a burst of 7-30 consecutive all-zero frames within a single second, all with identical zero content, while the actual session has ~111k tokens in use.

Solution

Filter all-zero usage frames at the data source layer in getContextWindowMetrics(). When used_percentage === 0 AND usedTokens is 0 or null, return null for all usage fields (usedTokens, contextLengthTokens, usedPercentage, remainingPercentage, cachedTokens, totalTokens). Keep windowSize, totalInputTokens, and totalOutputTokens intact since those are independently valid.

When the Context Bar widget receives null for contextLengthTokens, it falls through to tokenMetrics.contextLength (parsed from the session transcript JSONL file), which contains the real token count. This eliminates the 0k flash without any caching or state management overhead.

Why this approach

• No new files or caches: Uses the existing transcript-based fallback path that already works
• Minimal change: 19 lines of logic + 60 lines of tests, all in context-window.ts
• Data source filtering: Fixes the problem at the root rather than patching each consumer
• Preserves legitimate zero: If used_percentage is 0 but current_usage.input_tokens > 0, the data is preserved (not a true all-zero frame)

Test plan

• New unit tests: all-zero frame with current_usage object, scalar current_usage=0, and non-zero usage with used_percentage=0
• Existing tests pass (6/6 in context-window, 1304/1304 full suite)
• Manual testing: deployed locally, confirmed 0k flash no longer appears while progress bar maintains correct display via transcript fallback