Fix admonitions by declaring Zensical's default Markdown extensions

[LukasOro]

Problem

Paragraphs written as !!! note were rendering as literal text instead of as callouts, on introduction.md, get-started.md and mappings.md.

The Markdown was fine. The cause is in how Zensical resolves extension config (zensical/config.py):

config.get("markdown_extensions", DEFAULT_MARKDOWN_EXTENSIONS)

That is a fallback, not a merge. Zensical ships a default extension set that includes admonition, but the moment a project declares any markdown_extensions the whole default set is discarded and only the declared ones stay active. zensical.toml declared 8, so the other 14 defaults, admonition among them, were silently switched off.

Change

1. Declare the defaults explicitly. zensical.toml now lists the full effective extension set, with the inherited defaults fenced between BEGIN/END comments so a reader can tell upstream values apart from this project's own choices. The header explains the replace-not-merge behaviour and flags that the block needs re-checking whenever ZENSICAL_VERSION moves in the Makefile.

pymdownx.superfences (mermaid fence) and toc (permalink) are customized, so they sit in the project-specific block below instead: TOML forbids defining the same table twice. The defaults block names them so they aren't mistaken for non-defaults.

2. Disable pymdownx.smartsymbols (second commit). Restoring the defaults also restored this one, which rewrites (c), (r), (tm), +/-, -->, =/= and fractions like 1/2 wherever they appear in prose. On a spec site those sequences tend to be real content: it turned the (c) of an (a)/(b)/(c) enumeration in use-cases.md into a copyright sign. Because the substitution is silent, the next occurrence would be just as easy to miss. The entry stays in the defaults block commented out, with the reasoning beside it, so it reads as off-on-purpose rather than forgotten.

Effect on the built site

Diffed all 23 generated pages against a pre-change build. Beyond the three admonitions now rendering, exactly one other thing changed: pymdownx.highlight restoring class="language-… highlight" and per-line __span / __codelineno anchors on code blocks, across 11 pages. That is the markup Material's code copy/select and line-linking features expect, and it had been suppressed. No other prose was altered.

Verification

Full zensical@0.0.46 build --clean, matching the pinned version in the Makefile:

• 3/3 admonitions render; no literal !!! left in any generated page
• no smartsymbols glyphs in the output; the (a)/(b)/(c) enumeration reads correctly
• 24 tabbed blocks and the mermaid diagram unaffected
• site/spec/index.html byte-identical to before

Note for the reviewer

The (c\) escape added to docs/use-cases.md in the first commit is redundant once the second commit lands. It was left in place because it renders identically and re-protects the sentence if smartsymbols is ever switched back on, but it is easy to drop if you would rather have the plainer source.