feat: add missing builtins — fields(), system(), raw() — and embed hover docs

[SchoolyB]

Summary

The following builtins exist in the language spec (STANDARD.md §9.1, §3.1.11) but are missing from the LSP's BUILTINS array and/or DOCS object:

Builtin	Needs Completion	Needs Hover Docs
fields(instance) -> [string]	Yes	Yes
system(command string) -> int	Yes	Yes
raw(variable) -> ^T	Yes	Yes
embed(path string) -> string	No (already in BUILTINS)	Yes (missing from DOCS)

Details

• fields() — Returns field names of a struct as [string] in declaration order. Accepts struct instances and pointers to structs.
• system() — Runs a shell command and returns exit code. Returns -1 if killed by signal.
• raw() — Like addr(), but returns an unsafe raw pointer with no nil-check or const-source write protection.
• embed() — Already in the BUILTINS array for completion, but has no hover docs entry in the DOCS object.

Scope

• Add fields, system, raw to the BUILTINS array in completion.ts
• Add hover doc entries for all four in the DOCS object in hover.ts

[SchoolyB]

Implemented in 91f9533. Two corrections to the description:

embed already had hover docs. The issue lists it as "needs hover docs — missing from DOCS", but the DOCS entry was already present in gray-data.ts. No change was needed there. Only fields, system, and raw were genuinely missing, and all three are now in the BUILTINS array with hover docs.

One file the scope missed: the Zed highlight query at zed-extension/languages/gray/highlights.scm enumerates builtins in a #match? predicate, and that list was missing fields, system, raw, and embed. Without updating it, the new builtins would complete and hover correctly but render as plain identifiers in Zed. All four were added.

Verified afterwards that every entry in BUILTINS now resolves to a DOCS entry.