fix(compiler/phronesis-ast): move stranded #![forbid(unsafe_code)] to… (#63)

… crate top

`#![forbid(unsafe_code)]` is a crate-level *inner* attribute but was
left mid-file (line 34, between a doc comment and `struct Span`) — the
residue of a botched merge resolution. Rust rejects an inner attribute
there ("the inner attribute doesn't annotate this struct"), so
`phronesis-ast` failed to compile and its unit tests never ran.

Moved the attribute to the top of the file (after the SPDX header,
before the module docs). `cargo build` succeeds and `cargo test` passes
21/21. SPDX header left unchanged.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: hyperpolymath