fix(chunker): add Solidity language support

[dvcdsys]

What

Add full Solidity (.sol) support to the indexer: language detection on
both server and CLI, plus a chunker registry entry that wires the
tree-sitter Solidity grammar (already shipped by gotreesitter) to
contract / interface / library / struct / enum / event / function /
modifier / constructor node kinds.

Why

Solidity files were detected as unknown language and indexed as plain
text via the sliding-window fallback — no symbols, no contract /
function navigation, no cix def / cix refs. Worse, on the
server-side repoindexer path (GitHub-repo clones), .sol files were
skipped entirely because language resolved to "".

After this fix .sol files behave like every other first-class
language: structural chunks, symbol names, references, semantic search.

How

Three coordinated changes — language detection lives in two places by
design (CLI sends language= on file payloads, server falls back to its
own detection when missing), so both need the mapping:

• server/internal/langdetect/langdetect.go — .sol → "solidity"
• server/internal/chunker/chunker.go — new registry entry mapping
  grammars.SolidityLanguage to:
  • class: contract_declaration, library_declaration
  • type: interface_declaration, struct_declaration, enum_declaration, event_definition
  • function: function_definition, modifier_definition, constructor_definition, fallback_receive_definition
• cli/internal/discovery/language.go — same .sol → "solidity"
  in the CLI's own extension map

Node kinds were verified by parsing a real contract against the
gotreesitter Solidity grammar and reading the actual AST node types
(not guessed from the language reference).

Per CLAUDE.md the CLI and server release on independent tag streams,
but a shared contract change like this lands in one PR so neither side
drifts.

Tests

• New TestChunkFile_Solidity verifies that a sample contract yields a
  class chunk for Token, a function chunk for transfer, etc.,
  with correct parent linkage.
• TestRegistry_NodeNamesMatchAST fixture extended with a Solidity
  snippet so the configured node names stay in sync with the grammar
  (and any future grammar bump fails loudly).
• TestDetect (langdetect) gets a Token.sol → solidity case.

Manual verification on a sample contract: 10 symbols extracted
(contract, library, interface, struct, enum, event, function, modifier,
method) where before everything came out as a single block chunk with
no symbol name.

Type of change

• Bug fix
• New feature

Checklist

• go test ./internal/chunker/ ./internal/langdetect/ — green
• go build ./... — green on both server and CLI modules
• gofmt clean on all edited files
• No secrets or API keys committed

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