Add MySQL support to Lib\Db via multiple simultaneous named connections

Redesigns Lib\Db from a single global connection to a config-driven map
of named connections (config['db_connections']), each independently
lazy-connected, each with its own optional migrations_dir and its own
schema_migrations table. A sidecar can use more than one connection in
the same request (e.g. Db::query(...) against SQLite alongside
Db::query(..., 'legacy') against MySQL) — sidecars have full access to
any Lib\ class, so nothing stops a request from needing two databases
at once, which the original single-driver spec didn't account for.

Db::query()/Db::connection() both default to the 'default' connection
name so the common single-database case is unchanged at the call site.
Supersedes the flat db_driver/db_path/db_migrations_dir keys shipped in
the SQLite groundwork commit (a3b9967) — no downstream consumers yet,
so no migration path needed.

db_connections needed one deliberate exception to the project's usual
shallow config-merge rule: merged one level deeper, by connection name,
so App/config.php adding a connection doesn't delete 'default'. Verified
end-to-end against a real local MariaDB instance running alongside the
existing SQLite connection, which caught a real ordering bug in the
initial merge implementation (capturing defaults after they'd already
been overwritten) before it shipped.

Closes the "MySQL support" backlog item in novaconium/ISSUES.md.

README.md

@@ -16,12 +16,12 @@ A tiny, Hugo-flavored PHP micro-framework. Routes are directories on disk, pages
 - **Dark/light theme toggle** — a nav button flips a `data-theme` attribute (persisted to `localStorage`) that swaps every color via CSS custom properties; both palettes live in `App/sass/_colors.sass`, same override mechanism as everything else.
 - **Self-hosted spam prevention & form validation** — `Lib\SpamGuard`, a reusable class for any form: a CSS-hidden honeypot field plus a submission-timing check, no external CAPTCHA service, site key, or outbound API call. Pairs with `Lib\FormValidator` (accumulating required-field/email/length checks) and `Lib\Validate` (the underlying validation primitives — email, length, phone, postal/zip, spam-word checks). All three ship in `novaconium/lib/`, demonstrated on the contact form.
 - **Form security by default** — `Lib\Input`, a cleaning accessor for `$_POST`/`$_GET` (defense-in-depth against HTML/script injection, not a substitute for parameterized queries), and `Lib\Csrf`, standalone session-token CSRF protection called directly from a sidecar. Both ship in `novaconium/lib/`, wired into the contact form, `/admin/clear-cache`, and `/admin/password-hash`.
-- **SQLite database, zero setup** — `Lib\Db`, a thin PDO wrapper (no ORM) with a plain-SQL migration convention (`App/migrations/*.sql`, applied automatically on first use, or via `php novaconium/bin/migrate.php`). Data lives in a project-owned top-level `data/` directory, outside both `public/` and `novaconium/`. See `/admin/docs/database`.
+- **SQLite/MySQL database, zero setup** — `Lib\Db`, a thin PDO wrapper (no ORM) supporting multiple named connections open at once — e.g. this site's own SQLite data plus a MySQL connection to a legacy database, usable in the same sidecar — each with its own plain-SQL migration convention, applied automatically on first use or via `php novaconium/bin/migrate.php`. SQLite data lives in a project-owned top-level `data/` directory, outside both `public/` and `novaconium/`. See `/admin/docs/database`.
 - **No build step, no Composer** — clone it, point Apache (or `php -S`) at `public/`, and it runs. Twig is vendored as source; see `/admin/docs/upgrading-twig` for upgrading it.
 
 ## Getting started
 
-**Requirements:** PHP 8.1+ (uses `readonly` constructor-promoted properties) with the `pdo_sqlite` extension (bundled with PHP, just needs to be enabled — no separate install), and, for production, Apache with `mod_rewrite` and `AllowOverride All`.
+**Requirements:** PHP 8.1+ (uses `readonly` constructor-promoted properties) with the `pdo_sqlite` extension (bundled with PHP, just needs to be enabled — no separate install; add `pdo_mysql` too if using a MySQL connection), and, for production, Apache with `mod_rewrite` and `AllowOverride All`.
 
 ### Run it locally (no Apache needed)