Document the bundled test-data tarball in tutorial + reference

Following Harrison's PR #12 review: the standalone first-run tutorial
did not link the SPIDER-format EOS tables that the nightly CI fetches
from the GitHub release page. New users had to dig into the workflow
yaml to find the URL.

Adds a "Quick start: bundled test data" block to:
* docs/Tutorials/firstrun.md (under the "Locate or stage the EOS
  tables" step), showing the exact curl + tar + ARAGOG_TEST_EOS_DIR
  setup the nightly CI uses.
* docs/Reference/data.md (the canonical EOS-format reference).

Both blocks point at the same release-asset URL and instruct users
to set ARAGOG_TEST_EOS_DIR so the gated test suite picks it up.

Author: timlichtenberg

docs/Reference/data.md

@@ -19,6 +19,19 @@ The directory passed to `EntropySolver.from_file(eos_dir=...)` (or referenced th
 | `solidus_P-S.dat` | 2-column $(P, S)$ | Solidus entropy at each pressure |
 | `liquidus_P-S.dat` | 2-column $(P, S)$ | Liquidus entropy at each pressure |
 
+### Quick start: bundled test data
+
+A small SPIDER-format set is published as a release asset and used by the nightly CI. It is the fastest way to follow the [first-run tutorial](../Tutorials/firstrun.md) without a full PROTEUS / PALEOS install:
+
+```bash
+mkdir -p /tmp/aragog-test-data
+curl -sL -o /tmp/spider_eos.tar.gz \
+    https://github.com/FormingWorlds/aragog/releases/download/test-data-v1/spider_eos_test_data.tar.gz
+tar xzf /tmp/spider_eos.tar.gz -C /tmp/aragog-test-data/
+```
+
+The unpacked directory `/tmp/aragog-test-data/spider_eos/` is a drop-in `eos_dir`. Set `ARAGOG_TEST_EOS_DIR=/tmp/aragog-test-data/spider_eos` so the gated unit + smoke tests pick it up automatically.
+
 ### File layout
 
 The 2D tables follow SPIDER's text format: a header followed by a flat list of three-column $(P, S, Q)$ rows on a regular pressure-entropy grid.

docs/Tutorials/firstrun.md

@@ -28,7 +28,17 @@ Aragog's entropy solver requires a directory containing the following files:
 | `adiabat_temp_grad_solid.dat`, `adiabat_temp_grad_melt.dat` | 2D P-S grid | $(\partial T/\partial P)_S$ |
 | `solidus_P-S.dat`, `liquidus_P-S.dat` | 2-column $(P, S)$ | Phase-boundary entropy at each pressure |
 
-In a PROTEUS coupled run these tables are produced from the configured PALEOS or Wolf-Bower P-T file by the PROTEUS wrapper. For standalone work, point `eos_dir` at any directory containing this set.
+For a quick standalone walkthrough you can fetch a small SPIDER-format set bundled as a release asset:
+
+```bash
+mkdir -p /tmp/aragog-test-data
+curl -sL -o /tmp/spider_eos.tar.gz \
+    https://github.com/FormingWorlds/aragog/releases/download/test-data-v1/spider_eos_test_data.tar.gz
+tar xzf /tmp/spider_eos.tar.gz -C /tmp/aragog-test-data/
+export ARAGOG_TEST_EOS_DIR=/tmp/aragog-test-data/spider_eos
+```
+
+The same tarball is what the nightly CI populates before the smoke + slow tiers run. In a PROTEUS coupled run these tables are produced from the configured PALEOS or Wolf-Bower P-T file by the PROTEUS wrapper. For standalone work, point `eos_dir` at any directory containing this set.
 
 ## 2. Scaffold a configuration