feat(php-shim): implement fopen-family stream API on PhpResource

Redesign PhpResource into a real stream handle (File/Memory backing with
tracked position, eof, closed state) and unify the whole fopen family
(fopen/fwrite/fread/fgets/fgetc/feof/fclose/ftell/fseek/rewind/fstat/
ftruncate/fflush and stream_get_contents/stream_copy_to_stream) on
&PhpResource, replacing the split PhpMixed/PhpResource APIs and their
todo!() stubs. fopen now returns Result; read functions stay String for
now (TODO(phase-e) to move to byte strings).

Propagate the signatures through callers: Process stdout/stderr, Cursor
input, curl header/body handles (extracted into typed maps keyed by job
id), Filesystem copy/safe_copy/files_are_equal, BufferIO, error_handler,
platform, perforce, zip. The proc_open pipe paths cannot carry a
PhpResource in a PhpMixed list, so they are left as todo!() with notes.

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