feat(letsplot): implement line-stepwise (#2640)

## Implementation: `line-stepwise` - letsplot

Implements the **letsplot** version of `line-stepwise`.

**File:** `plots/line-stepwise/implementations/letsplot.py`

---
:robot: *[impl-generate
workflow](https://github.com/MarkusNeusinger/pyplots/actions/runs/20594566810)*

---------

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Co-authored-by: github-actions[bot] <41898282+github-actions[bot]@users.noreply.github.com>

Author: github-actions[bot]

plots/line-stepwise/implementations/letsplot.py

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+""" pyplots.ai
+line-stepwise: Step Line Plot
+Library: letsplot 4.8.2 | Python 3.13.11
+Quality: 92/100 | Created: 2025-12-30
+"""
+
+import numpy as np
+import pandas as pd
+from lets_plot import *  # noqa: F403, F401
+
+
+LetsPlot.setup_html()
+
+# Data - Server Response Time Monitor (discrete state changes)
+np.random.seed(42)
+hours = np.arange(0, 24)
+
+# Response time that changes in steps (server performance states)
+base_response = 50
+response_times = [base_response]
+
+for i in range(1, 24):
+    # Random step changes at certain hours
+    if i in [6, 9, 12, 15, 18, 21]:
+        change = np.random.choice([-20, -10, 10, 20, 30])
+        new_val = max(20, min(150, response_times[-1] + change))
+        response_times.append(new_val)
+    else:
+        response_times.append(response_times[-1])
+
+response_times = np.array(response_times)
+
+df = pd.DataFrame({"hour": hours, "response_time": response_times})
+
+# Plot
+plot = (
+    ggplot(df, aes(x="hour", y="response_time"))
+    + geom_step(color="#306998", size=2, direction="hv")
+    + geom_point(color="#FFD43B", size=5, shape=21, fill="#FFD43B", stroke=1.5)
+    + labs(x="Hour of Day", y="Response Time (ms)", title="line-stepwise · letsplot · pyplots.ai")
+    + scale_x_continuous(breaks=list(range(0, 25, 3)))
+    + theme_minimal()
+    + theme(
+        plot_title=element_text(size=24, face="bold"),
+        axis_title=element_text(size=20),
+        axis_text=element_text(size=16),
+        panel_grid_major=element_line(color="#CCCCCC", size=0.5),
+        panel_grid_minor=element_blank(),
+    )
+    + ggsize(1600, 900)
+)
+
+# Save PNG (scale 3x for 4800 x 2700 px)
+ggsave(plot, "plot.png", scale=3)
+
+# Save HTML for interactive version
+ggsave(plot, "plot.html")

plots/line-stepwise/metadata/letsplot.yaml

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+library: letsplot
+specification_id: line-stepwise
+created: '2025-12-30T10:43:40Z'
+updated: '2025-12-30T10:51:52Z'
+generated_by: claude-opus-4-5-20251101
+workflow_run: 20594566810
+issue: 0
+python_version: 3.13.11
+library_version: 4.8.2
+preview_url: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/letsplot/plot.png
+preview_thumb: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/letsplot/plot_thumb.png
+preview_html: https://storage.googleapis.com/pyplots-images/plots/line-stepwise/letsplot/plot.html
+quality_score: 92
+review:
+  strengths:
+  - Excellent step line visualization with clear horizontal-then-vertical transitions
+    using direction=hv
+  - Points overlay on steps helps identify exact data points at each hour
+  - Good color scheme with Python-inspired blue and yellow that is colorblind accessible
+  - Realistic server response time monitoring scenario that demonstrates discrete
+    state changes
+  - Proper title format following spec-id · library · pyplots.ai convention
+  - Clean KISS code structure with reproducible random seed
+  - Both PNG and HTML outputs generated correctly
+  weaknesses:
+  - Grid line color could be slightly more subtle
+  - Points at every hour are slightly redundant - could highlight only the change
+    points
+  - Data pattern is somewhat predictable with changes only at specific scheduled hours