VSCode extension: add hover documentation for IEC data types

[michaeldistel]

Context

The VS Code extension (introduced in #41) provides diagnostics but has no hover support. Hovering over a built-in IEC type like INT, TIME, or LREAL shows nothing. This is the smallest useful hover feature and has zero dependency on the compiler pipeline or symbol tables.

Details

Add a hover provider to the language server that responds when the cursor is on a built-in IEC 61131-3 data type name. The response should show:

• Full type name and category (e.g. Double Integer)
• Value range or valid values
• Size in bits
• Literal syntax where applicable (TIME, DATE, etc.)

Types to cover: BOOL, BYTE, WORD, DWORD, LWORD, SINT, INT, DINT, LINT, USINT, UINT, UDINT, ULINT, REAL, LREAL, TIME, LTIME, DATE, TIME_OF_DAY, TOD, DATE_AND_TIME, DT, STRING, WSTRING, CHAR, WCHAR

Implementation

• Add vscode-extension/server/src/hover-provider.ts with the type hover map and a getHoverForWord function
• Wire hoverProvider: true into server capabilities in server.ts
• Add connection.onHover handler in server.ts that extracts the word at cursor and delegates to the hover provider
• Unit test in vscode-extension/tests/

Acceptance

• Hovering any IEC data type in a .st file shows a markdown tooltip with type info
• Hovering a non-type word returns null (no tooltip)
• Unit tests cover all 27 types and the null case
• Compiler test suite still passes