fix: apply streaming deduplication to project cost, prediction, and heatmap

[long-910]

Summary

The requestId deduplication introduced in 0.5.1 (#32) only covered the global status-bar cost path (readJsonlFile / readAllUsage). Three other features each had their own ad-hoc JSONL parsing loop with no deduplication, so they still counted every streamed content-block line and inflated their figures 2–4× in agentic sessions:

• Project-level cost (src/data/projectCost.ts) — status bar suffix and dashboard Project card
• Burn rate / prediction (src/data/prediction.ts) — also skews rate-limit and budget exhaustion estimates
• Usage heatmap (src/webview/heatmap.ts) — daily and hourly aggregates

Changes

• Added a shared deduplicated reader to jsonlReader.ts:

  export interface UsageEntry { timestamp: number; usage: TokenUsage; cwd?: string }
  export async function readUsageEntries(filePath: string): Promise<UsageEntry[]>

  It builds on the existing readJsonlFile dedupe (by requestId, falling back to message.id) and additionally validates timestamps and normalizes all four usage fields.
• Rewired readAllUsage, projectCost.ts, prediction.ts, and webview/heatmap.ts onto it, deleting the three duplicated parsing loops.
• docs/DATA.md: new "Streaming Deduplication (applies to ALL readers)" section so future features go through the shared reader.
• CHANGELOG.md: Unreleased → Fixed entry.

Tests

• New readUsageEntries suite: dedupe by shared requestId, field normalization + ms timestamp parsing, skipping invalid entries, missing-file graceful degradation.
• Fixed a time-of-day flake in the heatmap suite: aggregates costs for active days used "N hours ago" offsets that cross midnight when CI runs before 03:00 local (it was failing in this session's container at 02:15 UTC). Timestamps are now anchored to local start-of-today.
• npm run lint ✅, npm run compile ✅, unit suites (jsonlReader / prediction / heatmap / cache, 34 tests) ✅ via mocha. The @vscode/test-electron runner cannot download VS Code in this sandbox; please rely on CI for the extension-host suites.

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https://claude.ai/code/session_012ZLCLQeudf1LQBnBSyyr4f

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