Extract the remaining languages into their own packages

[janrito]

This finishes the language-plugin split: every language still in core moves
out into its own rbtr-lang-* package, so core no longer bundles a grammar
it isn't asked to use.

The pilot PRs below packaged SQL and Bash and proved the mechanism; the rest
— around twenty languages — still lived in core, which hard-depended on every
tree-sitter-* grammar. So pip install rbtr pulled every grammar wheel
whether or not you indexed that language, languages could not be versioned on
their own, and their tests piled into shared files in core that only grew.
After this PR the eight default languages (bash, c, cpp, html, javascript,
markdown, python, rst) are required dependencies, so a bare install still
indexes them, and every other language is an optional extra that pays for its
grammar only when asked:

pip install rbtr            # the eight defaults
pip install rbtr[go,vue]    # add Go and Vue
pip install rbtr[all]       # every language

Each package is a workspace member that registers its languages through the
existing rbtr.languages entry point and owns its own sample-and-snapshot
tests; core registers nothing directly. Most languages move unchanged —
extraction is byte-identical to core.

Two deliberate exceptions carry real extraction changes. JSON, TOML, YAML,
and HCL now key their entries as config_key rather than doc_section,
because they are data, not documentation: a JSON object key that was a
doc_section chunk is now a config_key chunk of the same name. CSS likewise
moves its rule sets, @media, and @keyframes to class — a named
collection of declarations, like a table — and @charset to config_key.

Vue is the awkward case: its grammar ships inside the heavy
tree-sitter-language-pack, yet Svelte and Vue are the same kind of
single-file component and share one chunker. Rather than push that pack onto
everyone who only wants Svelte, SFC support is two packages —
rbtr-lang-svelte holds the shared machinery and the Svelte grammar, and a
thin rbtr-lang-vue depends on it for just the Vue grammar — so
rbtr[svelte] stays light and the pack arrives only with rbtr[vue].

The .scm query language is renamed from query to tree_sitter_query,
whose old id collided with rbtr's own search query. And now that every
language owns its tests, core's shared per-language test files are retired;
what remains is the engine's own tests, driven by synthetic registrations and
two default languages — C for exterior (leading-comment) docs, Python for
interior docstrings.

Each package proves itself by running its sample project through the real
extraction pipeline and snapshotting the result, so a plain move shows an
identical snapshot and an intended change shows a kind-only diff. The
orchestrator's multi-language dedup and per-language version-gating tests,
which had used the Svelte sample, are retargeted to a Markdown-embeds-Python
fixture so core's own tests lean only on default languages.

Two fixes ride along: the Bash plugin stops claiming Makefile and
Dockerfile — they are not shell, and claiming them suppressed the
searchable plaintext fallback — and load_query is cached so a query shared
by two registrations is read once.

This sits at the top of the stack, on the surface reshape and the extraction
fold below.