Extract buildDisplaySeries from rCpnlChart into 05b-pnl.js

[heyitsStylez]

Summary

Extracts the three inline series-preparation steps from rCpnlChart in 07-render-charts.js into a standalone pure function buildDisplaySeries in 05b-pnl.js, alongside computePnl. rCpnlChart is now a single call site.

The three absorbed steps:

• Zero-baseline prepend — anchors the chart at 0 on the first trade date
• Period-filter slice — cuts to 1M / 3M window with carry-forward baseline from the last point before the cutoff
• Today-extension — appends today's date as the final point so the line reaches now

Dead variable removed: cutoff (Date object), allSeries, lastBefore, inPeriod, todayStr — all absorbed into the function. The today Date object is replaced by today() (the existing util that returns an ISO string).

Interface:

buildDisplaySeries(series, period, today) → [{date, val}]
// series: realisedSeries from computePnl (raw cumulative, no baseline yet)
// period: '1M' | '3M' | 'ALL'
// today:  ISO date string e.g. '2026-05-16'

buildDisplaySeries added to the dual-export footer in 05b-pnl.js and to the 05b-pnl.js node in docs/architecture.md.

Test plan

4 new unit tests appended to test/unit/pnl.test.js:

• empty input returns empty — guard case

• ALL prepends zero-baseline and appends today — full happy path

• 1M with no in-range points returns flat [{cutoff,lastVal},{today,lastVal}] — all trades before window

• baseline carry-forward is last point before the cutoff — correct baseline selection when some trades are in-window

• npm test — 108/108 pass

• python3 build.py --check — passes

• Cumulative Realised P&L hero chart renders correctly for all three period toggles (1M / 3M / ALL) — behaviour unchanged, logic only moved

Closes #57

🤖 Generated with Claude Code

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