Polish docs nav and homepage badges

After the rename PR introduced nested subsection groups in the
left nav (the new topical grouping under Examples), the existing
nav CSS was no longer sufficient. This commit:

- Hides the redundant tab label inside the sidebar (the active
  tab name is already shown above the content via
  ``navigation.tabs``; Material was rendering it again as a
  ``.md-nav__title`` at the top of the level-1 nav).
- Adds a 1px vertical guide line on sub-sections that have
  children, via a ``::before`` pseudo-element on the level-2 nav.
  Bottom-stops 5px short of the nav's lower edge. Excludes
  Getting Started (whose single ``index.md`` child got rolled up
  by ``navigation.indexes`` leaving its level-2 nav empty).
- Shifts sub-section labels and their children's UL 10px LEFT
  to tighten the layout while leaving the guide line in place.
- Adds a padding-left override on label-form and direct-link-form
  ``.md-nav__link`` items so they align horizontally with the
  container-form labels. Material renders Documentation
  sub-sections as ``<div class="md-nav__container"><a>``, which
  matches the base padding rule twice (once on the outer div,
  once on the inner a), giving the text 0.75rem of layered
  offset. Examples sub-sections (``<label>``) and Patterns pages
  (``<a>``) match the rule once, so without this override their
  text would sit 7.5px left of the Documentation equivalents.

Also adds a Python-version shield to the docs homepage,
mirroring the README's shield: img.shields.io/python/required-
version-toml reads pyproject.toml's ``requires-python`` directly
so the badge auto-updates if the floor moves.

Verified across all three tabs via headless Chromium: all
sub-section / top-level page text lands at 14.5px from the
level-1 nav's left edge.

Author: chris-colinsky

docs/index.md

@@ -11,6 +11,7 @@ buy-in from every node.
 
 [![PyPI](https://img.shields.io/pypi/v/openarmature.svg?color=blue)](https://pypi.org/project/openarmature/){target="_blank" rel="noopener"}
 [![spec](https://img.shields.io/badge/dynamic/toml?url=https://raw.githubusercontent.com/LunarCommand/openarmature-python/main/pyproject.toml&query=%24.tool.openarmature.spec_version&label=spec&color=9D4EDD)](https://github.com/LunarCommand/openarmature-spec){target="_blank" rel="noopener"}
+[![python](https://img.shields.io/python/required-version-toml?tomlFilePath=https://raw.githubusercontent.com/LunarCommand/openarmature-python/main/pyproject.toml&label=python&color=blue)](https://pypi.org/project/openarmature/){target="_blank" rel="noopener"}
 
 [Get started](getting-started/index.md){ .md-button .md-button--primary }
 [View on GitHub](https://github.com/LunarCommand/openarmature-python){ .md-button target="_blank" rel="noopener" }

docs/stylesheets/extra.css

@@ -200,14 +200,110 @@
 /* Indent nested nav items so section hierarchy reads at a glance.
  * Material's ``navigation.sections`` layout renders section labels
  * and their children flush-left at the same horizontal position;
- * fine for short sections but hard to scan when one runs long (the
- * Examples section has 11 entries). A small left-pad on the nested
- * links shows the hierarchy without altering typography. Scoped to
- * the primary nav so the right-hand TOC sidebar is untouched. */
+ * a small left-pad on the nested links shows the hierarchy without
+ * altering typography. Scoped to the primary nav so the right-hand
+ * TOC sidebar is untouched. */
 .md-nav--primary .md-nav .md-nav__link {
   padding-left: 0.375rem;
 }
 
+/* Hide the active tab's title inside the sidebar. The top tabs
+ * (Documentation / Examples / Patterns) are shown above the
+ * content via ``navigation.tabs``; Material also renders the
+ * active tab's label as a ``.md-nav__title`` at the top of the
+ * sidebar, which is redundant. The level-1 nav is the tab's
+ * content; hiding its ``.md-nav__title`` drops the repeated
+ * label cleanly. */
+.md-nav--primary .md-nav[data-md-level="1"] > .md-nav__title {
+  display: none;
+}
+
+/* Vertical guide line + extra indent next to the *children* of an
+ * expandable sub-section group (Concepts / Model Providers /
+ * Reference under Documentation; Foundations / Composition /
+ * etc. under Examples). Material renders the section's toggle in
+ * two different shapes:
+ *
+ *   - ``<label class="md-nav__link">Composition</label>`` when the
+ *     section has no ``index.md`` of its own (Examples' topical
+ *     groups are like this).
+ *   - ``<div class="md-nav__link md-nav__container">`` when the
+ *     section has its own ``index.md`` (Documentation's Concepts /
+ *     Model Providers / Reference are like this, because
+ *     ``navigation.indexes`` wires the section's index page in via
+ *     a container that wraps both the index link and the toggle).
+ *
+ * Keying off either shape directly would also catch Getting Started
+ * (which renders the container shape too, since ``navigation.indexes``
+ * rolled its single ``index.md`` child up into the parent and left
+ * the level-2 nav empty). Keying instead off "has at least one
+ * direct child in the nested level-2 nav" cleanly skips Getting
+ * Started (empty nested nav) while matching every real
+ * multi-child sub-section in both tabs. Patterns' top-level pages
+ * have no sub-section nesting at all, so no line is drawn there. */
+/* Position the children's nav and shift the children's UL 10px
+ * LEFT, applied to ALL sub-sections under the level-1 nav so child
+ * links align horizontally regardless of whether the parent
+ * sub-section has the vertical guide line (which is itself scoped
+ * separately to multi-child sub-sections only). */
+.md-nav--primary .md-nav[data-md-level="1"] > .md-nav__list > .md-nav__item > .md-nav {
+  position: relative;
+  margin-left: 0.25rem;
+}
+
+.md-nav--primary .md-nav[data-md-level="1"] > .md-nav__list > .md-nav__item > .md-nav > .md-nav__list {
+  margin-left: -0.625rem;
+}
+
+/* Shift the sub-section label (the ``<label>`` or
+ * ``<div class="md-nav__container">`` wrapper) 10px LEFT to
+ * tighten the layout, while leaving the children's nav (with its
+ * vertical guide line) in place. */
+.md-nav--primary .md-nav[data-md-level="1"] > .md-nav__list > .md-nav__item > .md-nav__link {
+  margin-left: -0.625rem;
+}
+
+/* Align label-form labels (Examples: Foundations, Composition,
+ * etc.) and direct-link-form items (Patterns: Parameterized entry
+ * point, Tool dispatch as node, etc.) with container-form labels
+ * (Documentation: Concepts, Model Providers, Reference). The base
+ * padding rule (.md-nav--primary .md-nav .md-nav__link
+ * { padding-left: 0.375rem }) applies to ANY element with
+ * .md-nav__link class. In Documentation, the section toggle is
+ * rendered as <div class="md-nav__container"> wrapping
+ * <a class="md-nav__link">, and the base rule applies to BOTH the
+ * outer div AND the inner a, giving the text 0.75rem of layered
+ * left offset. In Examples (<label class="md-nav__link">) and
+ * Patterns (<a class="md-nav__link"> directly), the rule applies
+ * once and the text only gets 0.375rem of offset; result is text
+ * 7.5px left of the Documentation equivalent. This rule overrides
+ * the single-application forms' padding to 0.75rem so all
+ * sub-section / top-level page text lands at the same X column.
+ * ``:not(.md-nav__container)`` excludes Documentation's div wrapper
+ * so its layered offset stays intact. */
+.md-nav--primary .md-nav[data-md-level="1"] > .md-nav__list > .md-nav__item > .md-nav__link:not(.md-nav__container) {
+  padding-left: 0.75rem;
+}
+
+/* The vertical guide line, rendered as a ::before pseudo-element
+ * (so it can stop 5px short of the nav's bottom edge via
+ * ``top: 0; bottom: 5px``). Scoped to any sub-section whose nested
+ * nav has at least one child (the ``:has(> .md-nav > .md-nav__list
+ * > .md-nav__item)`` predicate); Getting Started's nested nav is
+ * empty (single ``index.md`` child got rolled up into the parent
+ * link via ``navigation.indexes``) so it doesn't match. A plain
+ * border would always cover the full element height; the
+ * pseudo-element gives us precise control over both ends. */
+.md-nav--primary .md-nav[data-md-level="1"] > .md-nav__list > .md-nav__item:has(> .md-nav > .md-nav__list > .md-nav__item) > .md-nav::before {
+  content: "";
+  position: absolute;
+  left: 0;
+  top: 0;
+  bottom: 5px;
+  width: 1px;
+  background: var(--md-default-fg-color--lightest);
+}
+
 [data-md-color-scheme="slate"] .md-header,
 [data-md-color-scheme="slate"] .md-footer-meta {
   background-color: var(--md-footer-bg-color);