refactor(ffi-linkage)!: Phase 9.0 — delete the hash_foreign token (src is foreign-free)

Per user directive (total purge): remove the hash_foreign token entirely rather than
keep it for a friendly deprecation message. Deleted: the token enum (token.zig), the
lexer keyword entry + directive-list mention + lex test (lexer.zig), the 4 parser
rejection sites + 2 lookahead clauses + the runtime-class prefix #foreign peek arm
(parser.zig), and the lsp completion arm (server.zig). '#foreign' now lexes as an
invalid '#' token → a generic 'expected ;' parse error (no migration hint — the
accepted UX cost of zero-foreign). Deleted examples/1176-diagnostics-foreign-removed
(its purpose, the friendly rejection, no longer exists).

src/ now contains ZERO 'foreign' (case-insensitive). Suite green (645 corpus / 443
unit, 0 failed). Remaining for the 9.4 gate: issues/*.md prose + example filenames.

examples/1176-diagnostics-foreign-removed.sx

@@ -1,10 +0,0 @@
-// Phase 8 (FFI-linkage) cutover: the prefix `#foreign` linkage directive has
-// been removed. A declaration imports an external C symbol via the postfix
-// `extern` keyword (or defines + exposes one via `export`). The parser rejects
-// `#foreign` with a clear migration message instead of routing it onto `extern`.
-//
-// Expected: one error on the `#foreign` token; exit 1.
-
-abs_c :: (n: i32) -> i32 #foreign;
-
-main :: () -> i32 { 0 }

examples/expected/1176-diagnostics-foreign-removed.exit

@@ -1 +0,0 @@
-1

examples/expected/1176-diagnostics-foreign-removed.stderr

@@ -1,5 +0,0 @@
-error: `#foreign` has been removed; use the postfix `extern` (import) / `export` (define) linkage keyword instead
-  --> examples/1176-diagnostics-foreign-removed.sx:8:26
-   |
- 8 | abs_c :: (n: i32) -> i32 #foreign;
-   |                          ^^^^^^^^