docs: cover help popover, +/- nav indicators, render_tool_header

User-facing docs (USER-GUIDE en+es, README en+es):
- New short paragraph under §3.1 GUI noting the in-tool Help button
  on every detail page, what it contains (When to use / Steps /
  Examples / Tip), and that content lives in tools.<id>.help_md.
- One-line note in the README tool tables pointing at the same.
- Mention the sidebar +/- nav indicators replacing Streamlit's
  default Material Symbols chevron.

Developer docs:
- DEVELOPER: new "Tool page header" subsection documenting
  render_tool_header(tool_id), the help_md markdown skeleton, and
  the fallback to help.missing_body when a tool's help is absent.
  Update i18n authoring rules to list help.* keys and the per-tool
  help_md field alongside name/description/page_title/page_caption.
- TECHNICAL: new §10c documenting the sidebar nav indicator swap —
  CSS in _HIDE_CHROME_CSS plus _SWAP_NAV_SECTION_INDICATOR_JS
  injected through the hide_streamlit_chrome() iframe bundle.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/TECHNICAL.md

@@ -242,6 +242,15 @@ The GUI uses an in-house, JSON-backed translation layer at `src/i18n/`. **No** `
 
 **Why not gettext**: zero compiled artifacts in the PyInstaller bundle, no build step before tests run, no `.po`/`.mo` round-trip for translators (anyone can edit JSON), and the same lookup works in unit tests without process state. Locked in because the surface won't grow large enough to need the alternative, and the alternative breaks the "drop a file, run pytest, ship" loop.
 
+## 10c. GUI chrome — sidebar nav indicator swap
+
+Streamlit's `st.Page`-driven sidebar renders section headers with a Material Symbols ligature (`expand_more` / `expand_less`). The header element is not a button and carries no `aria-expanded`, so a pure-CSS swap can't follow open/closed state. We replace the glyph with plain typographic `+` / `−` (U+2212) via JS:
+
+- **CSS** (`components/_legacy.py`, `_HIDE_CHROME_CSS`) drops the Material Symbols font on `[data-testid="stIconMaterial"]` inside `[data-testid="stNavSectionHeader"]` so the rewritten character renders as normal text rather than re-resolving as an icon name.
+- **JS** (`_SWAP_NAV_SECTION_INDICATOR_JS`) walks each section header, reads the icon's text node, and rewrites `expand_more` → `+` / `expand_less` → `−`. A MutationObserver re-runs the swap when Streamlit re-renders the sidebar (RAF-throttled so a burst of mutations is one swap).
+
+The script ships through the same component-iframe bundle as the brand injector and upload-button rename inside `hide_streamlit_chrome()` — one iframe per page, three DOM mutations.
+
 ## 11. Per-script functional specs
 
 Specs live in this section as scripts enter active build. Each follows the Tier 1/2/3 structure with explicit strategic framing (what's the market gap given some of this is free elsewhere).