Move php-magical-constants to php-keywords.el

[zonuexe]

Continues the php.el decomposition. Small and self-contained.

Why

php-keywords.el calls itself "a single source of truth for PHP keywords", but php.el kept a vocabulary list of its own under a ;;; PHP Keywords heading — so a reader asking where PHP's keywords live got two answers. This moves the list to the module that owns the vocabulary, which makes that claim true.

The name does not change

php-magical-constants stays php-magical-constants. It is a long-standing public symbol and both major modes build their php-magical-constant font-lock rule straight from it. Keeping the name is also exactly what #815 did — php-core.el holds php-executable, php-in-string-p and php-base-mode under their own names — so:

• no obsolete alias is needed,
• no caller has to change,
• (require 'php) behaves exactly as before, because php.el requires php-keywords.el and therefore still exposes it.

(An earlier sketch had this renamed to php-keywords-magic-constants with an alias. Dropped — it bought nothing and would have made the file's naming more mixed, since everything else there is php-keywords--*.)

Verification

• The defconst is moved byte-for-byte; diff against master confirms it is identical.
• After (require 'php), php-magical-constants still holds the same 8 constants.
• php-keywords.el still requires nothing, so the dependency arrow keeps pointing the natural way (vocabulary ← everything else), with no cycle.
• Byte-compiles with no new warnings; full suite green.

Not moved

php-re-token-symbols stays in php.el. Its only consumer is php.el's own token scanner, and it is a bare regexp with no list form, so it does not fit this file's NAME / NAME-re convention.