Add the native Stack language server

[konojunya]

Summary

• add stack lsp, a bounded LSP 3.18 stdio server with initialization, shutdown, position-encoding negotiation, incremental document synchronization, cancellation bookkeeping, and versioned diagnostics
• adapt the pinned compiler language-intelligence API to completion, hover, and hierarchical document symbols, and adapt the pinned engine formatter to standard text edits
• document editor setup, ownership, failure boundaries, and resource limits; include the command in release metadata checks
• pin the final compiler and engine revisions without adding a new third-party dependency or bundled vendor asset

Verification

• cargo +stable fmt --check
• cargo +stable test --locked
• cargo +stable clippy --all-targets --all-features --locked -- -D warnings
• RUSTDOCFLAGS=-D warnings cargo +stable doc --no-deps --locked
• pinned formatter and template conformance suites
• coverage thresholds: lines 90%, functions 95%, regions 90%
• cargo +1.85.0 test --locked
• cargo +1.85.0 clippy --all-targets --all-features --locked -- -D warnings
• release build, SVG validation, command metadata smoke, catalog/distribution/supply-chain validators
• Neovim 0.12 client smoke covering initialize, completion, hover, document symbols, formatting, incremental diagnostics, shutdown, and exit

Compatibility and security

Existing commands and exit contracts are unchanged. LSP input is limited to 8 MiB per message, 32 KiB of headers, 4 MiB per document, 64 open documents, and bounded request-ID caches. Malformed JSON is isolated to one response; unsafe framing and truncated input fail closed. Standard output remains protocol-only.