Processing and analysis code for remote-telescope imaging sessions

The scripts that processed the NGC 5128 session of 2026-07-21 previously
lived inside the data directory and addressed it with absolute paths.
Code and data are now separated: the code lives here, and a session is
located at runtime through the ASTRO_SESSION environment variable.

layout.py is what makes that work. It maps a FILENAME to the
subdirectory that file belongs in, using the same rules the session
directories are organised with, so a script can go on asking for
'master-Red.fit' or '_stars.npz' without any call site knowing the
directory structure. Anything unrecognised resolves to the session root,
which is visible and correctable rather than silently wrong.

restructure.py reorganises a flat session directory into that layout. It
is idempotent and dry-run by default.

The 50 session scripts are kept as they were run rather than tidied into
a library. They were written in sequence as the work went along, several
of them by parallel agents, and they show it - but they are the honest
provenance of a published set of results, and the productionised pipeline
should be able to reproduce those results exactly.

Verified before committing: all 51 files compile without warnings, and
verify_core.py, closeup.py and triptych.py were run end to end against
the reorganised session, correctly finding inputs across calibrated/,
stacks/masters/ and final/ and writing outputs back to the right places.

session-scripts/triptych.py

@@ -0,0 +1,62 @@
+"""Side-by-side of the three renderings, full field and a matched core crop.
+
+Same pixels in all three panels, so any difference is processing and nothing
+else.
+"""
+import os
+
+import matplotlib
+matplotlib.use("Agg")
+import matplotlib.pyplot as plt
+import numpy as np
+from PIL import Image
+
+import layout
+
+OUT = layout.SESSION
+# The finished renderings live in their own directory: they are the
+# deliverables, and keeping them apart from the masters, the
+# intermediates and the analysis figures makes it obvious which
+# files are meant to be looked at.
+FINAL = layout.path("final")
+PANELS = [
+    ("NGC5128-final-1-stacked.png", "1. Stacked + standard stretch",
+     "align, average, stretch - nothing else"),
+    ("NGC5128-final-2-processed.png", "2. Conventionally processed",
+     "gradient removal, colour balance, denoise"),
+    ("NGC5128-final-3-best.png", "3. Science-informed",
+     "halo-preserving background, solar-analogue colour,\n"
+     "core recovered, star-protected deconvolution"),
+]
+CROP = (1950, 1200, 2850, 1875)     # the dust lane, at full resolution
+
+fig, axes = plt.subplots(2, 3, figsize=(21, 11.5),
+                         gridspec_kw=dict(height_ratios=[1.35, 1]))
+fig.patch.set_facecolor("#111111")
+
+for col, (fname, title, sub) in enumerate(PANELS):
+    im = Image.open(layout.path(fname))
+    wide = im.copy()
+    wide.thumbnail((1500, 1500), Image.LANCZOS)
+    axes[0, col].imshow(np.asarray(wide))
+    axes[0, col].set_title(title, color="white", fontsize=15, pad=10)
+    axes[0, col].text(0.5, -0.055, sub, color="#9fb8d0", fontsize=10,
+                      ha="center", va="top", linespacing=1.4, wrap=True,
+                      transform=axes[0, col].transAxes)
+    axes[1, col].imshow(np.asarray(im.crop(CROP)))
+    axes[1, col].set_title("core, 1:1", color="#9fb8d0", fontsize=11, pad=6)
+    for row in (0, 1):
+        axes[row, col].set_xticks([])
+        axes[row, col].set_yticks([])
+        for s in axes[row, col].spines.values():
+            s.set_color("#333333")
+
+fig.suptitle("NGC 5128 (Centaurus A) - iTelescope T32, 2026-07-21 - "
+             "L 12x300s, RGB 4x300s each - the same data, three treatments",
+             color="white", fontsize=17, y=0.975)
+fig.tight_layout(rect=[0, 0.01, 1, 0.955])
+fig.subplots_adjust(hspace=0.16)
+path = layout.path("NGC5128-final-comparison.jpg")
+fig.savefig(path, dpi=100, facecolor=fig.get_facecolor(),
+            pil_kwargs={"quality": 92})
+print("wrote", path)