Language: reject catch (Exception $this) as Zend compile fatal (#32204)

[PurHur]

Summary

• catch (Exception $this) is a Zend compile-time fatal (Cannot re-assign $this); this compiler previously accepted the binding and ran (accepted, rc=0).
• Reject the catch variable in Compiler.php during Op\Stmt\TryCatch lowering, same path as param/global/static $this checks.
• php-src: Zend/zend_compile.c zend_compile_catch(). PHP: lib/Compiler.php assertNoThisAsCatchVariable(). No new runtime/ C.

Closes #32204

Test plan

• Issue repro VM + JIT (rc=255, Zend substring, no accepted)
• ./script/phpunit.sh --filter 'CatchThisCompileFatalTest|CatchThis32204|ReassignThisCompileTest' (13 tests)

Issue repro

./script/docker-exec.sh -- bash -lc 'php bin/vm.php test/repro/maintainer_gap_catch_this.php'

vm_rc=255
--- stdout ---
--- stderr ---
PHP Fatal error:  Cannot re-assign $this in /compiler/test/repro/maintainer_gap_catch_this.php on line 9

./script/docker-exec.sh -- bash -lc 'php bin/jit.php test/repro/maintainer_gap_catch_this.php'

jit_rc=255
--- stdout ---
--- stderr ---
PHP Fatal error:  Cannot re-assign $this in /compiler/test/repro/maintainer_gap_catch_this.php on line 9

Targeted PHPUnit

./script/phpunit.sh --filter 'CatchThisCompileFatalTest|CatchThis32204|ReassignThisCompileTest'

OK (13 tests, 24 assertions)

Not covered: full VMTest/JITTest suites (not green on master; compare by case name), ci-local.sh, north-star5-verify.

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