fix(chart): align road risers with same-day datapoint risers

[narthur]

What

Make buzz view's goal-progress chart match Beeminder's own graph at a derailment: the bright red line's vertical jump and the same-day datapoint's vertical riser now draw in the same column (overlapping), instead of several columns apart.

Why it looked wrong

Beeminder day-snaps both the road (roadall) knots and the datapoints onto one shared local-midnight grid before plotting, so a derail's road step and its #DERAIL datapoint line up. Buzz did neither:

• Road knots kept their raw mid-day (deadline-aligned) timestamps, while datapoints were bucketed to local midnight — so a road step and a same-day datapoint landed in different columns.
• The road was sampled at each column's left edge while datapoints bucket to the column at-or-before them (int truncation), adding a further one-column offset.

Result: on narthur/abed (a real derailment — road jumps 30 → 10091 on Jul 20, #DERAIL datapoint on the same day) the red riser sat 4 columns right of the blue one.

The fix

1. daysnapRoad — floor every parsed road segment boundary to local midnight (beebrain's daysnap equivalent), and recompute slopePerDay from the snapped boundaries.
2. Right-edge column sampling in roadValuesForTimeframe — sample each column at its right edge so a day-snapped knot's value change lands in the same column as a same-day datapoint node. The last column is clamped to endTime so the rightmost ("today") column never shows a value extrapolated past the window.

Before → after (real narthur/abed chart)

Before — stray red riser offset right of the blue one:

After — the red road riser is hidden directly behind the blue datapoint riser (same column; blue drawn on top), exactly as on Beeminder's graph:

   10149 ┤                                            ●──────●──────●──────────
    9134 ┤                                            │
     ...                                              │
       0 ┼●─────●─────●─────────────────────────●─────╯
          ┬                      ┬                      ┬                      ┬
          Jul 13              Jul 16                 Jul 20               Jul 23

(No stray red segment: the road step now coincides with the datapoint riser and is overdrawn by blue.)

Notes

• Right-edge sampling is a deliberate whole-road convention (matches the datapoint columns); at terminal resolution the sub-column shift on sloped roads is invisible — it only matters at vertical jumps, which is exactly where it makes the chart correct.
• This does not widen the chart window into the future — Beeminder's default graph also shows some future road, but that's a separate change.

Test plan

• go test ./... — green.
• New/updated tests: TestRoadStepAlignsWithSameDayDatapoint (road step and same-day datapoint node share a column, at two widths), TestDaysnapRoad (flooring, vertical-step equality, sub-day collapse, slopePerDay recompute), TestRoadValuesForTimeframe (right-edge sampling + last-column clamp regression case).
• Verified by rendering the real narthur/abed goal before/after.

🤖 Generated with Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Improved chart alignment by ensuring road changes and datapoints that occur on the same day appear in the same column.
  • Corrected road sampling at chart column boundaries.
  • Prevented chart values from extending beyond the selected timeframe.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 4358ebe3-bf83-470f-a11a-727e8cc992ca

📥 Commits

Reviewing files that changed from the base of the PR and between 3909fc0 and f35f82a.

📒 Files selected for processing (3)

• chart.go
• chart_test.go
• road_test.go

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📝 Walkthrough

Walkthrough

Changes

Goal chart alignment

Layer / File(s)	Summary
Road day snapping chart.go, chart_test.go	Road segment boundaries are floored to local midnight, slopes are recalculated, and chart rendering applies the snapped road. Tests cover day snapping and datapoint-step alignment.
Right-edge road sampling chart.go, road_test.go	Road values are sampled at column right edges, with the final sample clamped to endTime; tests update expected values and verify clamping.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Possibly related PRs

• PinePeakDigital/buzz#284: Introduced the goal chart road and sampling logic modified here.
• PinePeakDigital/buzz#312: Added local-midnight datapoint bucketing that this change aligns road rendering with.
• PinePeakDigital/buzz#324: Overlaps with the bright-red road sampling and value computation logic.

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly matches the main change: aligning road risers with same-day datapoint risers.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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• Commit unit tests in branch fix/chart-derail-riser-alignment

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