Sig language server - #24

`sigparser --lsp` speaks LSP over stdio and publishes the same errors a
generator run reports (syntax errors plus validate()), for the unsaved
contents of open buffers. Validation spans the whole sigs/ directory; a
file with syntax errors contributes its last good parse so other files'
references to it don't cascade into false errors.

LSP.cpp carries its own small JSON reader: importing cereal.json in a
second translation unit makes Main.cpp's make_map() crash with an access
violation (MSVC header-unit issue).

parse() is split into parse()/parse_text(); the per-file progress print
moves to the CLI path so stdout stays clean for the protocol.

The VSIX now also carries SigLanguageClient.dll (compiled by the script
with VS's own csc and assemblies) and the bundled sigparser.exe with its
DLL closure. SIG_LSP_SERVER overrides the bundled server path.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>